UC-NRLF 


B    3    7M1 


Studies  in  Heredity  as  Illustrated  by  the  Trichomes 

of  Species  and  Hybrids  of  Juglans,  Oenothera, 

Papaver,  and  Solanum 


BY 


WILLIAM  AUSTIN   CANNON 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 

1909 


CANNON 


Dorsal  and  ventral  surfaces  of  typical  leaves  of  Juglans  californica,  taken  from  a  tree  growing 
in  Santa  Rosa,  California.     One-third  natural  size. 


Studies  in  Heredity  as  Illustrated  by  the  Trichomes 

of  Species  and  Hybrids  of  Juglans,  Oenothera, 

Papaver,  and  Solanum 


BY 


WILLIAM  AUSTIN   CANNON 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 

1909 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  117 


THE  CORNMAN  PRINTING   COMPANY, 
CARLISLE,  PENNSYLVANIA. 


CONTENTS. 


Page 

Introduction i 

Scope  and  purpose  of  the  study l 

Pure  species  and  hybrids  examined 2 

Trichomes  of  Oenothera  lamarckiana  X  Oenothera  cruciata  and  of  the  parental 

lines 5 

Oenothera  lamarckiana 6 

Oenothera  cruciata 7 

Oenothera  lamarckiana  X  Oenothera  cruciata 9 

Comparison  of  trichomes  of  the  Oenotheras 10 

Trichomes  of  Papaver  hybrids  and  pure  species 12 

Trichomes  of  the  pure  species 12 

Papaver  somniferum  X  Papaver  orientale 13 

Papaver  somniferum  X  Papaver  pilosiim 13 

Trichomes  of  Solanum  villosum  X  Solanum  guinense  and  of  the  pure  species...  14 

Solanum  villosum 14 

Solanum  guinense 15 

Solanum  villosum  X  Solanum  guinense 15 

Pure  species  and  hybrids  of  Juglans 17 

Leaves  of  the  pure  species 17 

Juglans  californica 17 

Juglans  nigra  17 

Juglans  regia 18 

Trichomes  of  the  pure  species 18 

Juglans  californica 19 

Juglans  nigra 25 

Juglans  regia 30 

Comparison  of  leaves  and  trichomes  of  Juglans,  pure  species 34 

Hybrids  of  Juglans 35 

Leaves  of  the  hybrids 35 

Juglans  californica  X  Juglans  nigra 35 

Juglans  californica  X  Juglans  regia 35 

Trichomes  of  Juglans  californica  X  Juglans  nigra,  first  generation 37 

Trichomes  of  Juglans  californica  X  Juglans  regia,  first  generation 40 

Juglans  hybrids,  second  generation 42 

Leaves  of  the  hybrids 43 

Trichomes  of  the  hybrids 44 

Measurements  on  the  trichomes 52 

Juglans  californica  Y,  Juglans  nigra,  second  generation , 52 

Juglans  californica  X  Juglans  regia,  second  generation 55 

General  comparison  of  trichomes  in  pure  species  and  hybrids  of  Juglans 57 

Conclusions  and  results 61 

Summary 65 

in 


STUDIES  IN  HEREDITY  AS  ILLUSTRATED  BY  THE  TRICHOMES  OF  SPECIES 
AND  HYBRIDS  OF  JUGLANS,  OENOTHERA,  PAPAVER,  AND  SOLANUM. 


INTRODUCTION. 
SCOPE  AND  PURPOSE  OF  THE  STUDY. 

In  investigating-  the  structural  features  of  plants  two  or  more  methods 
of  approach,  each  especially  adapted  to  the  particular  aim  of  the  study, 
whatever  it  may  be,  may  be  employed.  The  entire  anatomy  of  a  few  forms 
may  be  worked  up,  and  appropriate  comparisons  instituted,  or,  on  the  other 
hand,  special  features  of  the  plant's  structure,  such  as  the  trichomes,  may 
be  minutely  studied,  both  in  the  mature  state  and  in  embryonic  condition,  in 
a  relatively  large  number  of  individuals  as  well  as  great  variety  of  species. 
Although  the  former  method  has  hitherto  been  the  one  chiefly  employed 
in  anatomical  studies  on  plant  hybrids,  it  seems  appropriate,  in  view  of 
the  present  tendency  to  regard  a  plant  as  a  complex  of  more  or  less  inde- 
pendent units,  to  institute  the  more  special  study.  Whatever  theories  may 
be  held,  this  manner  of  studying  structures  in  plants,  as  in  animals  has 
long  been  found,  has  several  advantages.  It  permits,  in  the  first  place, 
as  suggested  above,  an  examination  into  the  behavior  of  the  particular 
organ,  or  tissue,  in  a  much  larger  number  of  plants  than  might  otherwise 
be  practicable,  as,  in  the  present  instance,  the  trichomes  of  some  20  differ- 
ent kinds  of  plants,  and  many  more  individuals,  have  been  passed  under 
observation.  It  also  favors  an  investigation  into  the  variation  of  the  par- 
ticular structure,  as  well  as  into  the  relation  of  the  variation  to  the  organic 
and  the  physical  environment  of  the  tissue  as  possible  causal  factors  of 
variation.  Also,  not  only  the  mature  structures,  but  the  embryonic  stages 
likewise,  may  be  studied  and  compared,  so  that  developmental  stages  may 
easily  be  contrasted,  and,  in  favorable  material,  the  possible  origin  of  the 
structures  may  either  be  traced,  or  at  least  strongly  surmised.  And, 
finally,  what  is  particularly  important  in  a  study  on  heredity,  the  behavior 
of  the  same  structure  may  be  advantageously  observed  throughout  differ- 
ent generations.  For  these  and  other  reasons  the  particular  rather  than 
the  more  general  method  of  the  study  of  the  anatomy  of  the  hybrids  has 
been  adopted  in  the  present  investigation. 

In  selecting  plant  structures  for  comparative  study,  one  or  two  points 
were  kept  uppermost  in  mind.  All  of  the  plant  tissues  are  not  equally  favor- 

1 


2  HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 

able  for  such  an  investigation,  for  the  reason,  which  is  apparent,  that  they 
are  either  entirely  inclosed  by  other  tissues  or  so  intimately  related  to  such 
other  tissues  that  the  extent  as  well  as  the  manner  of  development  is 
largfely  conditioned  by  the  development  and  position  of  such  contiguous 
Itissues.  From  the  nature  of  things,  structures  so  enveloped  can,  to  a  very 
imited  degree  only,  whether  representing  "units"  or  not,  develop  as  inde- 
pendent organs.  However,  even  in  wholly  immersed  structures,  as  the 
conductive  system,  the  single-structure  method  of  approach  has  been  pro- 
ductive of  rich  results.  But  for  the  purpose  of  the  present  study  it  was 
essential  that  the  tissues  compared  be  as  independent  of  other  tissues  as 
possible,  and,  in  addition,  should  possess  well-marked  characteristics,  both 
as  mature  structures  and  in  embryonic  stages;  hence  it  was  decided  to 
select  for  the  comparative  study,  as  being  most  likely  to  give  satisfactory 
results,  the  trichomal  system. 

The  immediate  objects  of  the  investigation  were,  in  brief,  to  study  the 
origin,  development,  inheritance,  and  variation,  and,  so  far  as  practicable,  to 
observe  the  causes  of  variation  of  the  trichomes  in  several  pure  species  and 
the  hybrids  of  the  species.  In  addition,  it  was  desired  to  trace  the  behavior 
of  the  structures  in  as  many  plants  as  possible,  to  the  end  that  something 
exact  might  be  known  as  to  the  relation  between  such  tissues  and  unit- 
characters;  that  is,  to  be  more  specific,  it  was  proposed  to  learn,  for  ex- 
ample, whether  the  trichomal  system  as  a  whole  constitutes  a  single  unit, 
or  whether  each  separate  type  of  trichome  of  a  plant  is  to  be  considered  a 
distinct  and  independent  character. 

PURE  SPECIES  AND  HYBRIDS  EXAMINED. 

The  following  hybrids,  together  with  the  pure  lines,  were  studied: 
Juglans  calif ornica  X  Juglans  nigra,  Fn  F2,  and  F3;  Juglans  calif ornica  X  Jug- 
lans  regia,  Fj,  F^,  and  F3;  Oenothera  lamarckiana  X  Oenothera  cruciata,  F.J 
andFs;  Papaver  somniferum  X  Papaver  orientale ,  Ft;  Papaver somniferu m  X 
Papaver  pilosum,  F^  Solatium  villosum  X  Solanum  guinense,  Fx,  F2,  and 
later  generations. 

The  material  for  the  study  of  the  trichomes  of  the  Oenothera  hybrid  was 
collected  at  the  New  York  Botanical  Garden  in  August,  1906,  where  it  had 
been  formed  three  years  previously.  In  this  cross*  the  characters  of  the 
pollen  parent  {Oenothera  cruciata)  are  said  largely  to  dominate,  although 
none  appear  to  be  transmitted  unchanged.  Particularly  in  the  flower  there 
is  a  singular  blending  of  the  characters  of  both  parents ,  as ,  for  example ,  some 
of  the  hybrids  have  short  stamens,  lamarckiana-\\k&,  and  are  incapable  of 

*Mutants  and  Hybrids  of  the  Oenotheras,  by  D.  T.  Macdougal  and  others,  Carnegie 
Institution  of  Washington  Publications  24  and  81,  1905. 


INTRODUCTION.  3 

self-pollination,  and  others  have  long;  stamens,  critciata-like ,  and  may  be 
self -pollinated.  Thus  the  hybrid  does  not  in  any  character  revert  consist- 
ently to  either  parent.  This  is  an  example  of  the  first  cross  breeding  true. 

The  material  for  the  study  of  the  Jtiglans  hybrids  was  collected  at  Mr. 
Luther  Burbank's  experimental  grounds  in  Santa  Rosa  and  Sebastopol, 
California,  in  1907  and  1908,  where,  about  20  years  before,  Mr.  Burbank 
had  made  the  original  crosses. 

Juglans  calif ornica  X  Juglans  nigra  is  represented  by  one  tree  of  the  first 
generation  called  the  ' '  Royal , ' '  and  by  numerous  smaller  trees  of  the  sec- 
ond and  third  generations,  the  latter  called  the  "Beeson,"  in  the  nursery 
at  Sebastopol.  Plates  4,  5,  6,  and  9  illustrate,  though  very  inadequately, 
the  variability  of  the  leaves  of  the  first,  second,  and  third  generations  of 
this  hybrid  and  show  that  the  greatest  range  is  probably  to  be  found  in 
the  second  generation,  although  those  of  the  third  are  also  extremely 
variable.  The  young  trees  of  the  later  generations  exhibit  also  great 
differences  in  size,  as  variation  in  this  regard  exceeding  500  per  cent  has 
been  observed  between  those  of  like  age  and  growing  under  apparently 
identical  conditions. 

Juglans  calif  ornica  X  Juglans  regia  is  represented  by  several  trees  of  the 
first  generation,  known  as  the  "Paradox,"  in  Santa  Rosa,  and  by  num- 
erous second  and  third  generation  seedlings  at  Sebastopol.  Four  Paradox 
trees  are  growing  in  the  street  in  front  of  Mr.  Burbank's  residence.  Al- 
though only  17  years  from  the  seed,  these  trees  are  about  27  meters  in 
height  and  their  trunks  are  about  75  centimeters  in  diameter.  The  very 
luxuriant  character  of  the  tree  is  extended  to  the  leaves,  which  in  some 
instances  are  surprisingly  long  and  are  composed  of  leaflets  which  in  num- 
ber and  size  are  said  to  exceed  those  of  either  parent.  As  will  be  pre- 
sented somewhat  in  detail  later  in  this  paper,  the  leaves  are  also  very  vari- 
able in  surface  characters,  in  shape,  size,  and  contour.  Certain  of  these 
variations  in  the  first  and  second  generation  plants  are  shown  in  plates  7, 
8,  and  10. 

The  hybrids  Papaver  somniferum  X  Papaver  orientale  and  Papaver  somni- 
ferum  X  Papaver  pilosum  were  produced  by  Mr.  Burbank  and  are  now  grow- 
ing at  his  experimental  grounds  in  Santa  Rosa,  California.  The  parents 
of  these  hybrids,  natives  of  southeastern  Europe,  the  Caucasus  region,  and 
Asia  Minor,  are  either  annual  (somniferum)  or  perennial,  but  the  hybrids 
are  perennial.  Both  of  the  Papaver  hybrids  are  extremely  variable,  espe- 
cially as  regards  the  size,  shape,  and  texture  of  the  leaves  and  the  size, 
color,  and  form  of  the  flowers,  and  they  are  either  wholly  sterile  (/*.  som- 
niferum X  P.  orientale)  or  to  a  certain  degree  fertile.  On  account  of  the 
complete  or  partial  sterility  of  these  hybrids  only  the  first  generation  has 
been  available  for  study. 


4  HEREDITY   AS    ILLUSTRATED   BY   TRICHOMES. 

Solanum  villosum  X  Solarium  guinense  was  produced  by  Mr.  Burbank 
and  is  now  growing"  in  his  experimental  grounds  at  Santa  Rosa,  California. 
Solanum  guinense,  a  native  of  central  west  Africa,  is  a  perennial  of  bushy 
habit  of  growth,  and  bears  black  fruit  of  unpleasant  flavor;  Solanum  villo- 
siim,  from  Chile,  is  an  annual,  dwarfed  and  procumbent,  and  bears  clusters 
of  small,  hard,  and  green  berries.*  The  hybrid  is  a  low,  bushy  perennial, 
and  produces  berries  agreeable  to  the  taste,  the  flavor  of  which  recalls  that 
of  the  low-bush  blueberry  of  the  eastern  United  States.  The  hybrid  has 
not  been  known  to  revert,  although  its  culture,  begun  in  1897,  has  been 
carried  to  the  fifth  generation;  it,  therefore,  is  an  instance  of  the  breeding 
true  of  a  first  cross. 

The  material  for  study  of  all  hybrids  procured  from  Mr.  Burbank  was 
either  collected  by  the  writer  and  fixed  in  the  usual  manner,  or  was  put  up 
by  Dr.  G.  H.  Shull  in  weak  formaldehyde.  To  both  of  these  gentlemen 
thanks  are  due  for  the  preparation  of  material,  and  to  Mr.  Burbank,  who 
so  generously  gives  both  his  plants  and  his  time  for  the  forwarding  of  the 
study  of  plant  hybrids,  acknowledgment  is  especially  and  gratefully  made. 
Acknowledgment  should  also  be  made  to  the  directors  of  Hopkins  Seaside 
Laboratory,  Pacific  Grove,  California,  for  the  use  of  a  research-room  dur- 
ing the  summer  of  1907,  and  to  the  Carmel  Development  Company,  Carmel- 
by-the-Sea,  California,  who  provided  a  laboratory  which  was  used  during 
the  summer,  1908. 

•Year  Book,  Carnegie  Institution  of  Washington,  1907. 


TRICHOMES  OF  OENOTHERA  HYBRIDS  AND  PARENTAL  LINES. 


TRICHOMES  OF  OENOTHERA  LAMARCKIANA  X  OENOTHERA 
CRUCIATA  AND  OF  THE  PARENTAL  LINES. 

Three  types  of  trichomes  are  found  in  both  Oenothera  lamarckiana  and 
Oenothera  cruciata,  and  each  species  bears  all  three  types  (figf.  l).  These 
are  awn-  or  awl-shaped,  club-shaped,  and  pear-shaped,  and  are  referred 
to  as  such  in  the  following-  account  of  them.  All  of  the  trichomes  are 
unicellular. 


FIG.  1. — a  to  d,  Oenothera  lamarckiana;  etOff,O.  cruciata;  htoj,  O.  la- 
marckiana X  cruciata.  a,  pear-shaped  trichomes  from  the  stem 
(X  535);  6,  club-shaped  trichome  from  the  veins,  lower  leaf-surface 
( X  535);  c,  awn-shaped  trichome  from  ventral  surface  of  leaf,  be- 
tween veins  (X  &4);  d,  pear-shaped  trichomes  from  ventral  surface 
of  leaf,  between  veins  (X  362);  e,  awn-shaped  trichome  from  be- 
tween the  veins,  ventral  leaf-surface  (X  362);  /,  pear-shaped  tri- 
chomes from  ventral  surface  of  leaf,  between  veins;  g,  awn-shaped 
trichome  from  stem  (X362);  h,  giant  awn-shaped  trichome  from  3 
capsule  (X  84);  i,  pear-shaped  trichomes  from  ventral  leaf-surface 
(X  535);  j,  club-shaped  trichome  from  capsule  (X  362). 

The  awn-shaped  trichomes  of  the  older  leaves  are  lifeless.  They  vary 
in  length  to  a  marked  degree,  and  in  extreme  instances  rangre  from  71.4  p- 
to  1.8  mm.,  althoug-h  for  the  most  part  the  differences  in  length  lie  be- 
tween 200  /*  and  1.00  mm. 


6  HEREDITY   AS   ILLUSTRATED   BY   TRICHOMES. 

The  club-shaped  trichomes,  as  well  as  the  pear-shaped  ones,  are  gland- 
ular, and  hence  living".  Both  are  less  variable  in  size  than  the  awn-shaped 
trichomes  and  both  have  lengths  characteristic  of  each  species.  The  lead- 
ing" characters  of  the  glandular  trichomes  are  sufficiently  shown  in  fig.  1 
and  do  not  require  further  description  in  this  place. 

OENOTHERA  LAMARCKIANA. 

In  the  pure  parent  Oenothera  lamarckiana,  the  three  kinds  of  trichomes 
above  described  are  to  be  found  on  the  lower  surface  of  the  leaves,  on  the 
stem,  and  on  the  capsule.  The  variation  of  the  awn-shaped  trichome,  which 
was  referred  to  above,  may  be  shown  by  the  following  measurements, 
expressed  in  M:  ventral  surface  of  the  leaf,  163.8,  168,  191.2,  202.6,  222.2, 
223.1,  244.6,  248.8,  265.6,  306.6,  344.4,  399.0,  420.4.  The  trichomes  above 
measured  were  selected  at  random;  the  following  measurements  are  of  tri- 
chomes which  were  in  the  region  of  the  veins  of  the  leaf,  expressed  in  p-: 
126.0,  231.0,  239.4,  252.0,  252.0,  336.0,  399.0,  420.0,  453.6.  On  the  stem 
the  measurements  were  126.0,  159.6,  176.0,  269.6,  387.2,  440.0,  721.6, 
756.8,  844.8,  880.0,  915.0,  1144.0,  1196.8.  On  the  capsule  these  trichomes 
were  less  numerous  but  were  all  relatively  long,  as  the  following  measure- 
ments, in  P-,  indicate,  844.8,  897.6,  915.2,  922.8,  968.0,  1003.6,  1073.6, 
1073.6,  1144.0,  1196.8. 

The  club-shaped  trichomes  occur  also  on  the  ventral  surface  of  the  leaves , 
on  the  capsule ,  and  on  the  stem .  This  type  of  trichome  is  more  uniform  in 
size  in  any  given  area,  as  between  the  veins  or  on  the  veins  of  the  leaf, 
than  the  awn-shaped  trichome,  and  also  the  trichomes  of  different  areas 
are  more  nearly  the  same  size  than  those  of  the  other  (awn-shaped)  type. 
This  is  also  true,  and  perhaps  more  consistently  so,  of  the  pear-shaped 
trichomes,  the  measurements  of  which  will  be  given  below,  from  which 
circumstance  a  study  of  them  as  inheritable  qualities  is  of  value.  The  fol- 
lowing measurements,  in  p-,  were  made  of  the  club-shaped  trichomes  taken 
from  the  lower  leaf-surface  and  between  the  veins:  168.0,  176.4,  180.6, 
184.8,  189.0,  189.0,  197.4,  201.6,  210.0,  227.0,  231.2,  231.0,  247.8,  252.0, 
253.0.  Trichomes  on  the  veins  measured  as  follows,  in  p-:  168.0,  168.0, 
180.6,  189.0,  191.2,  201.6,  205.6,  211.0,  211.0,  231.0,  252.0.  Club-shaped 
trichomes  on  the  stem  had  the  following  lengths,  in  p-:  131.2,  172.2,  176.4, 
180.6,  180.6,  201.6,  210.0,  210.0,  222.6,  248.8.  On  the  capsule  these  tri- 
chomes measured  as  follows,  in  P<:  155.4,  163.8,  176.4,  189.0,  205.6,  211.0, 
222.6,  231.0,  235.2,  252.0,  252.0.  The  average  lengths  of  the  club-shaped 
trichomes  for  the  given  areas  are  as  follows:  lower  surface  of  leaf,  on  vein, 
200.08  p-;  between  veins,  209.2  p-;  on  stem,  193.4  p-;  on  capsule,  208.5  p-. 

Pear-shaped  trichomes  were  studied  on  the  ventral  surface  of  leaves,  on 
the  stem,  and  on  the  capsule.  The  following  measurements,  in  p-,  give  the 


TRICHOMES   OF   OENOTHERA   HYBRIDS   AND    PARENTAL    LINES. 


lengths  of  these  trichomes  for  the  different  areas  noted.  Ventral  surface 
of  leaf  ,  between  the  veins:  37.8,  37.8,  37.8,  39.9,  42.0  42.0,  42.0,  42.0,  42.0, 
46.2,  46.2,  46.2,  50.4,  50.4,  50.6;  on  the  veins,  37.8,  42.0,  46.2,  50.4,  50.4, 
54.6,  54.6,  54.6,  54.6,  54.6,  54.6,  63.0,  63.0,  67.2;  on  the  stems:  37.8,  37.8, 
39.9,39.9,42.0,42.0,  26.2,46.2,46.2,  50.4,  54.6,  54.6,163.0,67.2,67.2.  The 
pear-shaped  trichomes  were  not  so  abundant  on  the  capsules  as  elsewhere; 
9  trichomes  from  the  capsule  measured  as  follows,  in  /*:  37.8,  46.2,  48.3, 
50.4,  65.6,  58.8,  58.8,  58.8,  58.8.  The  averages  in  length  for  these  hairs 
are:  (l)  between  the  veins,  on  the  lower  surface  of  the  leaf,  43.5/*;  (2)  on 
the  veins,  53.4  p-;  (3)  on  the  stem,  54  p-;  and  (4)  on  the  capsule,  52.5  p-. 

In  comparing  the  relative  development  of  the  two  kinds  of  glandular  tri- 
chomes on  equivalent  areas  we  find  that  those  on  the  stem  are  rather  small 
and  that  those  on  the  capsule  and  on  the  veins  of  the  leaf  are  relatively 
large,  but  the  relation  of  the  size  of  the  two  trichomes  to  the  given  areas 
is  not  strictly  consistent.  For  example,  the  smallest  pear-shaped  trichomes 
are  to  be  found  between  the  veins  on  the  lower  surface  of  the  leaf,  while 
it  is  in  such  areas  that  the  longest  club-shaped  trichomes  occur.  This 
relation  is  shown  in  table  1. 

TABLE  1. — Relative  lengths  of  the  club-shaped  and  the  pear-shaped 
trichomes  of  Oenothera  lamarckiana. 


Where  found. 

Club- 
shaped. 

Pear- 
shaped. 

Leaf,  between  veins.... 
Leaf,  on  veins  

/"• 
209.2 
200.  8 

fj- 
43-9 

C-5  .A 

Stem  

ten.  4. 

CO.  1 

Capsule  

208.5 

62  .  <; 

OENOTHERA  CRUCIATA. 

The  three  forms  of  trichomes  which  are  to  be  found  in  Oenothera  lamarck- 
iana are  present  in  cruciata  also  (fig.  l).  In  form  and  in  other  charac- 
ters, the  trichomes  of  cruciata  are  like  those  of  the  other  species,  only  they 
are  consistently  smaller. 

The  awn-shaped  trichomes,  upon  the  lower  surface  of  the  leaves  and 
between  the  veins,  are  relatively  uniform  in  length  and  usually  straight. 
The  extremes  in  length  which  were  observed  were  71.4  ^  and  131.4  p.  On 
the  veins  the  trichomes  are  somewhat  more  variable  and  range  in  length 
between  71.4  /"•  and  399  p.  Mixed  with  the  shorter  awn-shaped  trichomes 
there  are  on  the  stem  numerous  giant  trichomes  of  the  same  character, 
save  as  to  size.  These  were  seen  as  long  as  1.3  mm.  The  giant  trichomes 
only,  of  this  general  type,  were  found  on  the  capsule,  where  they  attained  a 
length  as  great  as  1.8  mm. 


8 


HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 


The  club-shaped  trichomes  were  not  so  abundant  as  in  the  other  pure 
species.  None  were  found  on  the  lower  surface  of  the  leaf,  either  upon  or 
between  the  veins,  and  none  on  the  stem.  The  following-  measurements, 
in  p-.  were  made  of  club-shaped  trichomes  on  the  capsule:  96.6,  121.8, 
134.4,  134.4,  147.0,  163.8,  168.0.  The  average  length  was  138.0  j*. 

The  pear-shaped  trichomes  occur  on  the  lower  surface  of  the  leaves,  on 
the  stem,  and  on  the  capsule.  Other  than  being  somewhat  smaller  in  size 
they  are  like  those  of  Oenothera  lamarckiana.  Trichomes  between  the  veins 
on  the  lower  surface  of  the  leaves  were  found  to  have  the  following  lengths, 
in/*:  23.1,  25.2,  25.2,  29.4,  29.4,  29,4,  33.6,  33.6,  33.6,  33.6,  35.7,  35.7,  39.9, 
39.9,  42.0.  The  trichomes  between  the  veins  average  32.5  ^  in  length.  On 
the  veins  the  trichomes  gave  the  following'  measurements,  in  M:  25.2,  31.6, 
33.6,  33.6,  33.6,  33.6,  35.7,  37.8,  37.8,  37.8,  37.8,  39.9,  42.0,  42.0,  46.2. 
The  average  length  of  the  pear-shaped  trichomes  on  the  veins  was  found 
to  be  36.5  p-.  The  following  measurements,  in  p-,  were  derived  from  tri- 
chomes on  the  stem:  29.4,  29.4,  29.4,  31.5,  33.6,  33.6,  33.6,  33.6,  33.6, 
33.6,  33.6,  35.7,  35.7,  35.7.  The  trichomes  on  the  stem  average  33.4  p  in 
length.  On  the  capsule  the  measurements,  in  /*,  were:  27.3,  29.4,  29.4, 
31.5,  31.5,  31.5,  33.6,  33.6,  33.6,  33.6,  33.6,  35.7,  37.8,  37.8,  3.78.  The  pear- 
shaped  trichomes  of  the  capsule  average  33.2  p  in  length.  The  relation  of 
the  pear-shaped  and  club-shaped  trichomes  to  the  areas  where  they  occiir 
is  shown  in  table  2. 

TABLE  2. — Relation  of  the  trichomes  of  Oenothera  cruciata  to  the 
Position  on  the  plant  where  found. 


Where  found. 

Club- 
shaped. 

Pear- 
shaped. 

Leaf,  between  veins.... 
Leaf,  on  veins  

M 

Absent 
Absent 

M 
32-5 

16.  <; 

Stem  

Absent 

-5-5.4 

Capsule  

n8.o 

\\.  2 

In  comparing  the  lengths  of  the  pear-shaped  trichomes  of  the  two  species 
we  find  that  those  of  Oenothera  cruciata  are  smaller  wherever  found  than 
those  of  Oenothera  lamarckiana,  but  that  in  both  species  the  trichomes  are 
of  variable  length.  The  longest  trichomes,  with  hardly  an  exception,  occur 
in  the  region  of  the  veins  of  the  leaf,  and  the  shortest  occur  between  the 
veins  of  the  leaf.  In  both  species,  also,  the  trichomes  of  the  stem  and  of 
the  capsule  are,  as  a  rule,  intermediate  in  length.  The  absence  of  club- 
shaped  trichomes  from  the  leaf  and  the  stem  of  Oenothera  cruciata  precludes 
a  comparison  of  this  trichome  in  the  two  plants,  but  it  is  significant  that  the 
trichomes  of  this  type  on  the  capsule  of  Oenothera  cruciata  are  smaller  than 


TRICHOMES  OF  OENOTHERA  HYBRIDS   AND  PARENTAL  LINES. 


those  on  the  capsule  in  the  other  species,  as  might  be  expected  from  a  com- 
parison of  the  pear-shaped  trichomes.  The  awn-shaped  trichomes  are  so 
irregular  in  their  variation  that  a  comparison  of  them  would  be  meaningless. 

OENOTHERA  LAMARCKIANA  X  OENOTHERA  CRUCIATA 

The  three  sorts  of  trichomes  which  occur  in  both  pure  parents,  the  awn- 
shaped,  the  club-shaped,  and  the  pear-shaped,  are  to  be  found  in  the  hybrid 
also  (fig.  l).  The  trichomes,  except  as  to  size,  are  apparently  like  those 
of  the  parents;  they  are  intermediate  in  size.  The  figures  show  the  lead- 
ing characters  of  the  trichomes  and  a  further  description  of  them  in  this 
place  is  unnecessary. 

The  awn-shaped  trichomes  on  the  ventral  surface  of  the  leaf,  between  the 
veins,  measure  from  84.0  p-  to  323. Op-  long;  those  on  the  veins  range  from 
84.0  /*  to  1073.6  p-  in  length.  Awn-shaped  trichomes  from  the  stem  are  from 
96. 6 /*  to  1214. 4 /*;  those  on  the  capsule  are  all  of  the  long  kind  and  measure 
from  915.2  /*  to  1284.8  ^  in  length. 

The  club-shaped  trichomes  were  not  seen  on  the  lower  surface  of  the 
leaves  or  on  the  stem,  but  were  abundant  on  the  capsule.  The  following 
measurements,  in  /*,  were  made  of  such  trichomes  from  the  capsule:  163.8, 
168.0,  180.6,  193.2,  201.6,  205.8,  210.0,  222.6,  239.4,  243.6.  These  tri- 
chomes average  202-8  /«•  in  length. 

The  pear-shaped  trichomes  occur  on  the  lower  surface  of  the  leaf,  on  as 
well  as  between  the  veins,  on  the  stem  but  not  on  the  capsule.  Trichomes 
from  the  lower  surface  of  the  leaf,  between  the  veins,  measured  as  fol- 
lows, in/A:  29.4,  29.4,  29.4,  31.6,  33.6,  33.6,  35.7,  35.7,  37.8,  37.8,  37.8, 
42.0,  43.2;  on  the  veins,  33.6,  33.6,  35.7,  35.7,  37.8,  37.8,  42.0,  42.0,  42.0. 
42.0,  44.1,  44.1,  46.2,  50.4.  The  average  length  of  the  former  is  34.9  /*, 
of  the  latter  is  41.3  )"-.  The  following  measurements,  in  p,  were  obtained 
from  trichomes  on  the  stem:  33.6,  33.6,  35.7,  35.7,  37.8,  37.8,  42.0,  42.0, 
42.0,  42.0,  42.0,  44.1,  44.1,  46.2,  50.4.  The  average  length  of  the  pear- 
shaped  trichomes  on  the  stem  was  found  to  be  40.6  p. 

Table  3  gives  the  relative  lengths  of  the  trichomes  of  Oenothera  lamarck- 
iana  X  Oenoihera  cruciata. 

TABLE  3.—  Relative  lengths  of  club-shaped  and  pear-shaped  trichomes 
of  Oenothera  lamarckiana  X  Oenothera  cruciata. 


Where  found. 

Club- 
shaped. 

Pear- 
shaped. 

Leaf,  between  veins  
Leaf,  on  veins  

M 

Absent 
Absent 

M 
34-9 

41    "\ 

Stem  

Absent 

4O.6 

Capsule  

202    8 

Absent 

10  HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 

COMPARISON  OF  TRICHOMES  OF  THE  OENOTHERAS. 

Upon  comparing  the  extremes  in  length  of  the  trichomes  of  the  hybrid 
with  those  of  the  parents  we  find  (l )  that  the  shortest  pear-shaped  trichome 
in  Oenothera  cruciata  is  23.1  P-;  the  shortest  in  Oenothera  lamarckiana,  on  the 
other  hand,  is  37.8  /*,  while  the  shortest  in  the  cross  is  29.4  p,  and  (2)  that 
the  longest  pear-shaped  trichome  in  Oenothera  lamarckiana  is  67.2ft,  the 
longest  in  the  hybrid  is  50.4  p-,  and  in  Oenothera  cruciata  is  46.2  p-.  In  every 
instance  the  shortest  pear-shaped  trichomes  occur  on  the  lower  surface  of 
the  leaf  and  between  the  veins,  although  in  Oenothera  lamarckiana  short 
ones  occur  also  on  the  other  areas;  the  longest  are  to  be  found  on  the  veins, 
although  in  the  hybrid  long  ones  may  occur  on  the  stem  as  well.  As  will 
appear  repeatedly  in  this  study,  this  relation  of  the  length  of  trichome  to 
area  where  found  is  so  consistent  in  all  plants  that  there  may  be  some  com- 
mon underlying  cause,  as,  for  example,  nutritive  conditions,  which  induces 
the  differentiation.  At  any  rate,  the  variation  is  constant  and  must  be 
taken  into  account  in  the  comparison  of  the  trichomes  of  related  forms. 

Reference  to  tables  1,2,  and  3  will  show  that  in  every  instance  where 
analogous  trichomes  from  similar  areas  are  compared,  those  of  the  hy- 
brid are  intermediate  in  size,  those  of  Oenothera  lamarckiana  are  larger, 
and  those  of  Oenothera  cruciata  are  smaller.  In  every  instance  also,  the 
trichomes  of  the  hybrid  are  somewhat  less  than  one-half  the  sum  of  the 
other;  that  is,  they  are  not  average  in  measurement.  In  such  hybrids  as 
the  above,  when  both  parents  possess  identical,  or  practically  identical,  tri- 
chomes, whose  sole  apparent  difference  is  that  of  size,  we  would  perhaps 
expect  such  a  result.  This  would  be  based  on  one  of  at  least  two  grounds: 
First,  either  that  the  trichomes  of  the  hybrid  would  represent  the  equal  in- 
fluence of  both  parents,  as  McFarlane*  showed  long  ago  for  several  hybrids; 
or  second,  if  there  was  inconsistent  working  of  the  law  of  dominance  so 
that  a  portion  of  the  trichomes  would  show  reversion  to  one  and  a  portion 
to  the  other  parent  and  the  third  portion  be  indeterminate  in  this  regard, 
the  average  of  the  whole  would  be  intermediate.  So  far  as  the  present 
studies  would  indicate,  there  is  no  clear  proof  that  reversion  may  not  occur 
as  stipulated  in  the  second  alternative,  since  60  percent  of  the  pear-shaped 
trichomes  of  Oenothera  lamarckiana  come  within  the  range  of  size  exhibited 
by  these  types  in  the  hybrid,  and  80  per  cent  of  the  pear-shaped  trichomes 
of  Oenothera  cruciata  are  within  the  range  of  size  of  those  of  the  hybrid. 
But  it  seems  more  probable,  from  the  general  intermediate  condition  of 
the  hybrid,  that  the  trichomes  may  each  also  show  the  influence  of  each 
parent  in  approximately  equal  measure. 

*A  comparison  of  the  minute  structure  of  plant  hybrids  with  that  of  their  parents, 
and  its  bearing  on  biological  problems.  Trans.  Roy.  Soc.  Edinb.,  37:  203,  1892. 


TRICHOMES   OF   OENOTHERA   HYBRIDS   AND    PARENTAL    LINES. 


11 


A  study  of  the  range  of  variation  of  the  pear-shaped  trichomes  shows 
that  those  of  the  hybrid  in  nearly  every  instance  are  less  variable  than 
those  of  either  parent.  The  equations  in  table  4,  which  are  numbers  based 
on  the  extreme  lengths  of  the  trichomes,  present  the  relative  variability  in 
a  graphic  manner. 

TABLE  4. — Comparative  variability  of  pear-shaped  trichomes  of  the  Oenotheras. 


Where  found. 

O.  lamarck- 
iana. 

O.lamarckiana 
X  O.  cruciata. 

O.  cruciata. 

Leaf,  between  veins  
Leaf  on  veins  

149 

177 

146 

I  HO 

181 
181 

Stem  

167 

i  so 

121 

Average  

164 

148 

164 

The  distribution  of  the  trichomes  shows  the  influence  of  both  parents  in 
an  unequal  degree.  The  awn- shaped  trichomes  are  found  on  all  areas, 
/.  e.,  on  leaf,  stem,  and  capsule,  but  the  club-shaped  and  pear-shaped  tri- 
chomes are  variously  distributed.  The  former  occurs  on  all  of  the  areas 
in  Oenothera  lamarckiana,  but  only  on  the  capsule  in  Oenothera  cruciata  and 
the  hybrid.  The  distribution  of  the  pear-shaped  trichomes  is  much  more 
general  and  uniform;  they  occur  on  the  leaf  and  the  stem  in  parents  and 
the  hybrids,  and  on  the  capsule  in  Oenothera  cruciata,  but  are  not  present  on 
the  capsule  in  the  cross,  and  only  sparingly  so  on  the  capsule  in  Oenothera 
lamarckiana.  Where  there  are  several  kinds  of  trichomes  present  the 
distribution  of  each  kind  in  parents  and  hybrid  offspring  must  be  com- 
pared in  order  to  derive  any  exact  knowledge  of  the  influence  of  the  parents 
or  the  behavior  of  the  pubescent  covering  throughout  the  generations  of 
hybrids  studied.  It  is  doubtful  whether  the  quality  of  hairiness  in  plants 
can  rightly  be  considered  a  "unit-character,"  and  the  analysis  must  be 
carried  yet  further. 


12 


HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 


TRICHOMES  OF  PAPAVER  HYBRIDS  AND  THE  PURE  SPECIES. 
TRICHOMES  OF  THE  PURE  SPECIES. 

The  three  species  of  Papaver  which  were  studied  have  only  one  type  of 
trichome,  which  is  multicelltilar,  awn-shaped,  and  usually  slightly  curved 
(fig".  2).  The  trichomes  may  attain  a  length  so  great  as  7.0  mm.  (Papaver 
orientate),  although  they  usually  are  shorter  than  this. 


FIG.  2. — Trichomes  of  Papaver  pure  lines  and  hybrids.  Papaver  spmniferum:  a,  multicellu- 
lar  trichome  from  young  stem  (  x  44);  6,  detail  of  tip  of  trichome,  showing  appressed  dis- 
tal ends  of  the  superficial  cells  (  x  362).  Papaver  orientate:  c,  tip  of  trichome,  to  show  the 
projection  of  distal  ends  of  superficial  cells  (  X  362);  d,  detail  from  middle  portion  of  tri- 
chome (  X  362).  Papaver  pilosum:  e,  2  trichomes,  semi-diagrammatic,  to  illustrate  the 
projection  of  superficial  cells  (  x  84).  Papaver  somniferum  X  P.  orientate:  f,  detail  from 
middle  part  of  a  typical  trichome,  to  show  the  inconsistent  behavior  of  superficial  cells, 
only  a  portion  of  which  project  (  X362);  g,  tip  of  trichome  (  X  362).  Papaver  somniferum 
X  P-  pilosum:  h  and  i,  trichomes  from  a  leaf,  illustrating  relatively  prominent  projections 
of  superficial  cells  (  X  84). 

The  trichomes  have  a  curious  structure,  unlike  any  other  encountered  in 
the  course  of  this  investigation,  Their  growth  appears  to  be  indetermin- 
ate. As  a  direct  result  of  this  character  the  length  of  the  trichomes  is 
very  unequal,  so  that  comparative  study  of  them  from  this  standpoint  would 
be  without  value.  The  trichomes  end  in  a  single  much-elongated  cell 


TRICHOMES  OF  PAP  AVER  HYBRIDS  AND  PURE  SPECIES.  13 

(figf.  2,  ^).  Somewhat  away  from  the  tip  there  are  2  cells  in  cross-section, 
still  further  there  are  3  cells,  and  the  number  of  cells  of  the  cross-section 
increases  as  one  goes  towards  its  base.  At  the  base  the  trichome  expands 
suddenly,  so  that  in  longitudinal  section  it  is  conical.  All  of  the  cells  of 
the  trichome  are  elongated  in  a  direction  parallel  to  its  longer  axis.  The 
increase  in  length  takes  place  at  the  tip  of  the  trichome,  where  new  cells  are 
cut  off  by  somewhat  oblique  walls.  Growth  in  diameter  appears  to  occur 
by  longitudinal  divisions  of  the  inner  cells,  but  the  exact  sequence  of  these 
divisions  was  not  studied.  The  trichomes  of  the  three  species  are  similar 
in  form  and  size,  but  they  are  unlike  in  quality  of  roughness.  In  Papavcr 
orientate  and  Papaver  pilosum  the  distal  ends  of  the  superficial  cells  project 
beyond  the  general  surface  of  the  trichome  and  turn  out  at  a  rather  acute 
angle  (fig".  2,  c,  d,  and  e).  In  Papaver  somniferum,  however,  these  cells  did 
not  extend  beyond  the  general  surface,  with  the  effect  that  the  trichomes 
of  this  species  are  smooth  (fig.  2,  a). 

The  leaves,  stem,  and  flower-stalk  of  Papaver  orientate  and  of  Papaver 
pilosiim  are  well  clothed  with  trichomes;  but  Papaver  somniferum  is  prac- 
tically glabrous,  as  only  a  few  trichomes  were  observed  on  the  flower-stalk, 
and  none  on  stem  or  leaf. 

PAPAVER  SOMNIFERUM  X  PAPAVER  ORIENTALE. 

The  leaves  and  the  stem  of  the  hybrid  are  well  covered  with  trichomes, 
which  in  size  and  in  form  resemble  those  of  Papaver  orientate,  but  in  certain 
regards  they  are  unlike  the  trichomes  of  that  species.  They,  also,  are  dif- 
ferent from  the  trichomes  of  Papaver  somniferum,  and  in  each  instance  the 
variance  lies  in  the  character  of  the  peripheral  cells.  The  distal  ends  of 
the  peripheral  cells,  which  in  the  seed  parent  are  suppressed,  but  which 
project  with  much  regularity  in  the  pollen  parent,  in  the  hybrid  are  ex- 
tremely variable  in  their  behavior.  In  some  instances  they  do  not  project 
at  all,  in  others  the  projection  is  well  marked,  perhaps  accentuated,  and 
frequently  a  middle  condition  was  noted  (fig.  2,  /).  In  this  particular, 
therefore,  the  hybrid  is  fairly  intermediate  and  exhibits  the  influence  of 
both  parents. 

PAPAVER  SOMNIFERUM  X  PAPAVER  PILOSUM. 

The  hybrid  Papaver  somniferum  X  Papaver  pilosum  is  especially  well 
covered  with  trichomes  which  closely  resemble  those  of  the  other  Papaver 
hybrid  studied.  That  is,  in  one  character,  namely,  that  of  the  surface  of 
the  trichomes,  they  are  intermediate  between  the  pure  parents.  The  pro- 
jections of  the  superficial  cells  are,  if  anything,  even  more  irregular  than 
in  the  other  hybrid  (fig.  2,  h  and  z).  In  some  instances  the  projections 
are  suppressed  entirely,  in  others  they  are  exaggerated  and  the  trichome 
has  the  appearance  of  bearing  branches,  but  an  intermediate  condition  also 
occurs.  This  variation  is  suggested,  but  nowise  adequately  shown,  in  the 
accompanying  sketches. 


14 


HEREDITY   AS   ILLUSTRATED    BY  TRICHOMES. 


TRICHOMES  OF  SOLANUM  VILLOSUM  X  SOLANUM  GUINENSE 
AND  OF  THE  PURE  SPECIES. 

SOLANUM  VILLOSUM. 

There  are  2,  or  possibly  3,  kinds  of  trichomes  in  Solatium  villosum, 
all  of  which  are  multicellular  and  glandular  (fig-.  3,  a,  b,  and  c).  These 
for  convenience  are  in  this  paper  called  the  awn-shaped,  the  giant  awn- 
shaped,  and  the  big-headed  trichomes.  The  trichomes  were  observed  on 
the  stem,  on  the  lobes  of  the  calyx,  and  on  the  ventral  surface  of  the  leaves. 
On  the  stem  and  on  the  calyx  all  3  types  occurred,  with  no  apparent 
choice  of  position,  but  on  the  leaf  the  distribution  was  as  follows:  only  the 
awn-shaped  trichomes  occurred  between  the  veins;  all  were  found  on  the 
veins. 


PIG.  3. — Trichomes  of  Solatium  pure  species  and  hybrids.  Solanum 
villosum:  a,  awn-shaped  secreting  trichome  from  leaf,  region  of  vein; 
6,  giant  awn-shaped  secreting  trichome  from  region  of  midrib,  ven- 
tral leaf-surface;  c,  big-headed  glandular  trichomes  from  young  stem. 
Solanum  guinense:  d,  big-headed  glandular  trichomes  from  ventral 
leaf-surface;  e,  awn-shaped  non-secreting  trichome.  Solanum  villo- 
sum X  S.  guinenne:  f,  big-headed  glandular  trichome  from  ventral 
surface  of  leaf;  g,  big-headed  glandular  trichome.  (For  size  of  tri- 
chomes see  text;  reduced  one-half.) 


The  awn-shaped  trichomes  are  composed  of  a  single  row  of  cells,  usually 
5,  which  constitute  the  stalk,  and  a  single  terminal  cell,  which  is  some- 
what swollen  and  contains  coarsely  granular  matter.  The  stalk -cells  are 
provided  with  a  delicate  protoplasmic  lining  and  are  hyaline.  They  meas- 
ure from  176 p-  to  352  /"•  in  length,  with  the  greatest  number  about  193  p- 
long. 


TRICHOMES  OF  SOLANUM  HYBRIDS  AND  PURE  SPECIES.  15 

The  giant  awn-shaped  trichomes  are  different  from  the  awn-shaped  ones 
mainly  in  the  quality  of  size;  they  measure  from  792  /*  to  1.4  mm.  No  tri- 
chomes were  observed  of  the  awn- shaped  type  which  measured  longer 
than  the  longest  awn-shaped  trichomes,  352  /*,  or  shorter  than  the  shortest 
giant  awn-shaped  trichomes,  792  /*,  for  which  reason  the  giant  awn-shaped 
trichomes  ought  probably  to  be  considered  a  distinct  type. 

The  big-headed  trichome  is  somewhat  curved,  so  that  it  is  frequently 
closely  appressed  to  the  surface.  In  structure  it  is  composed  of  a  stalk  of 
2  cells  and  a  head  of  an  undetermined  number  of  cells,  probably  4.  The 
stalk-cells  are  hyaline;  the  cells  composing  the  head  are  densely  filled  with 
a  coarse  granular  substance.  These  trichomes  measure  from  63  p  to  75.6  p- 
in  length. 

SOLANUM  GUINENSE, 

Two  types  of  trichomes  were  observed  in  Solanum  guinense,  of  which 
one  is  glandular  and  similar  to  the  big-headed  trichome  of  Solanum  villosum, 
and  the  other  is  not  glandular  and  is  unlike  any  trichome  seen  in  the  other 
species  (fig.  3,  e} . 

The  trichomes  were  found  mainly  on  the  veins  of  the  ventral  surface  of 
young  leaves  and  on  young  stems;  the  older  stems  and  older  leaves  were 
usually  glabrous. 

The  big-headed  secreting  trichomes  were  either  straight  or  mostly  curved 
and  appressed  to  the  surface.  They  measure  from  67 . 2  p  to  71 .4  /*  in  length. 
As  in  structure,  size,  and  general  appearance  these  trichomes  are  similar 
to  the  analogous  ones  in  Solanum  villosum,  a  further  description  of  them  is 
unnecessary. 

The  non-secreting  trichome  (the  uncommon  type)  is  broadly  awn-shaped, 
is  usually  curved,  and  more  or  less  closely  appressed  to  the  surface.  It  is 
composed  of  3  or  4  cells,  which  rest  on  a  multicellular  base.  The  trichomes 
measure  from  239.4  /*  to  378.0  p-  in  length. 

SOLANUM  VILLOSUM  X  SOLANUM  GUINENSE. 

Two  types  of  trichomes  were  observed  in  the  hybrid  Solanum.  These 
were  in  all  respects  like  those  of  the  species  Solanum  guinense,  and  conse- 
quently were  the  big-headed  secreting  trichome  and  the  awn-shaped  non- 
secreting  trichome  (fig.  3,  e  and/).  The  trichomes  are  on  the  ventral  leaf- 
surface,  where  they  are  confined  to  the  veins.  The  big-headed  trichomes 
measure  from  67.2  p-  to  80 /*  in  length,  and  the  awn-shaped  trichomes  from 
252. 2  p.  to  316.8  /*  in  length. 

The  account  above  given  is  from  a  study  of  first-generation  plants. 
About  a  dozen  second-generation  plants  were  examined  also  and  a  condi- 
tion quite  like  that  observed  in  the  first  generation  was  found.  Measure- 
ments of  the  big-headed  secreting  trichomes  of  second-generation  hybrids 
showed  that  they  ranged  from  63.0  /*  to  71.4  /*  on  the  leaves,  and  71.4  p  to 


16 


HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 


75.6  p-  on  the  stem.     The  non-secreting  awn-shaped  trichomes  measured 
from  218.8  /*  to  316.8  p>  in  length. 

The  general  relation  of  the  trichomes  of  the  hybrid  and  the  pure  lines  of 
Solanum  are  given  graphically  in  table  5. 

TABLE  5.— Comparison  of  the  trichomes  of  Solanum  villosum   X   Solanum  guinense 

with  those  of  the  pure  species. 


Kind. 

S.  villosum. 

The  hybrid. 

S.  guinense. 

Awn-shaped,  glandular  

176  n  to  352  n  

Absent  

Absent. 

Giant  awn-shaped  .. 

792  /j.  to  1.4  mm... 

Absent  

Absent 

Big-headed  

67  M  to  7$  6  M  . 

67    2  /U  tO  80  fJi  

67  2  u.  to  7i  A  u 

Awn-shaped,  not  glandular.. 

Absent  

*(63^  to  71.  4  /*)... 
"(71.  4M  to  75-6^) 
252.2  /x.  to  316.  8  /* 

239.  4  //•  to  378  tt. 

*(2i8.8Mt03i6.8/u) 

*The  numbers  in  parenthesis  refer  to  hybrids  of  the  second  generation. 

The  data  at  hand  are  not  sufficient  either  in  amount  or  in  kind  to  permit 
the  tracing  of  the  influence  of  the  pure  lines  on  the  hybrid  offspring.  This 
much  is  apparent,  however:  the  trichomes  of  the  non-secreting  awn-shaped 
type  which  the  hybrid  inherits  from  Solanum  guinense  are  of  full,  not  of 
half  size,  as  MacFarlane  found  in  certain  hybrids  investigated  by  him 
where  unilateral  inheritance  also  obtained.* 


*MacFarlane,  /.  c. 


CANNON 


PLATE  2 


Dorsal  surface  of  leaves  of  Juglans  nigra,  which  was  growing  near  the  Sebastopol,  California, 
ranch  of  Luther  Burbank.  The  leaves  were  selected  to  show  the  extreme  variation  in 
size  and  number  of  leaflets,  and  size  of  leaves.  The  basal  pair  of  leaflets  in  the  figure  to 
the  right  are  only  partly  shown.  One-third  natural  size. 


PURE   SPECIES   AND   HYBRIDS    OF   JUGLANS.  17 

PURE  SPECIES  AND  HYBRIDS  OF  JUGLANS. 
LEAVES  OF  THE  PURE  SPECIES. 

The  English  walnut  and  the  eastern  black  walnut  are  common  in  culti- 
vation in  and  around  Santa  Rosa,  where  they  grow  thriftily.  The  Cali- 
fornia walnut  is  native  at  Santa  Rosa  and  is  also  extensively  cultivated  as 
a  shade  tree  in  the  city,  as  elsewhere  throughout  the  State.  Material  for 
the  study  of  the  California  walnut  (Jug tans  californica)  and  the  English 
walnut  (Juglans  regia)  was  obtained  from  trees  16  and  13  meters  in  height, 
respectively,  which  are  growing  on  the  grounds  of  two  of  Mr.  Burbank's 
neighbors  in  Santa  Rosa.  Material  of  the  third  type  (Juglans  nigra)  was 
collected  from  a  second-growth  tree,  about  10  meters  high,  which  was 
growing  by  the  roadside  between  the  town  of  Sebastopol  and  the  Burbank 
ranch  near  that  place.  The  collections  were  made  in  May,  1907,  and  May, 
1908.  It  is  not  known  whether  the  trees  from  which  the  collections  of 
material  were  made  were  the  actual  parents  of  the  original  crosses  or  not. 
It  is  assumed  in  this  study  that  the  alternative  is  the  case. 

JUGLANS  CALIFORNICA. 

The  leaves  of  Juglans  californica  bear  from  9  to  10  pairs  of  leaflets  and 
1  terminal  one  (plate  l),  which  is  frequently  the  smallest  one  of  the  leaf. 
The  leaflets  are  ovate  lanceolate  and  taper  gradually  to  the  tip;  the  base 
is  abrupt  and  even  cordate;  that  of  the  terminal  leaflet  is  attenuate.  They 
are  somewhat  roughened  on  the  upper  surface  and  are  irregularly  serrate, 
with  rather  obtuse  teeth.  A  close  study  of  the  leaflets  would  show  con- 
siderable variation,  especially  in  size  and  in  form,  although  the  extent  of 
the  variation  has  not  been  especially  examined. 

In  May,  1907,  when  the  first  collection  of  leaves,  which  were  exclusively 
studied,  was  made,  they  were  fresh-green  and  free  from  fungus;  in  the 
following  September,  when  the  photographs  were  made,  the  upper  surface 
was  badly  infested  with  a  black  fungus  which  to  an  extent  shows  in  the 
plate . 

JUGLANS  NIGRA. 

The  leaves  of  Juglans  nigra  are  extremely  variable,  particularly  as  to 
size  and  number  of  leaflets,  which  number  from  6  to  10  or  more  pairs  and 
which  are  irregularly  disposed  on  the  rachis  (plate  2).  In  addition  to  the 
variation  of  the  leaves  as  a  whole,  the  leaflets,  also,  are  far  from  uniform 
and  in  outline  range  from  nearly  cuneate  to  ovate;  the  terminal  one  is 
usually  the  smallest.  They  taper  gradually  to  an  attenuated  apex.  The 
bases  are  abrupt,  but  usually  they  are  not  cordate.  The  leaflets  are  serrate, 
with  fairly  regular  and  sharply  pointed  teeth;  the  ventral  surface,  in  tex- 
ture, is  generally  smooth, 


18 


HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 


JUGLANS   REGIA. 

The  leaves  of  Juglans  regia  are  composed  of  from  2  to  3  pairs  of  leaflets 
and  a  terminal  one.  The  leaflets  usually  grade  in  size  from  the  basal  ones 
to  the  terminal  one,  which  is  largest.  They  are  broadly  ovate  and  taper 
rather  abruptly  to  the  apex,  as  well  as  to  the  base.  The  base  of  the 
terminal  leaflet  is  somewhat  attenuated.  The  margin  of  the  leaflets  is 
entire;  the  upper  surface  of  the  leaflets  is  minutely  roughened  or  nearly 
smooth,  with  the  veins  projecting  somewhat  above  it  (plate  3). 

Table  6  presents  in  a  condensed  form  the  leading  characters  of  the  leaves 
of  the  3  species  of  Juglans. 

TABLE  6. 


Leaves  with  5  to  7  leaflets J.  regia. 

Leaves  with  19  to  21  leaflets...     J.  californica 

Leaves  with  13  to  21  leaflets J.  nigra 

Leaflets  serrate J.  californica.     J.  nigra 

Leaflets  entire J.  regia. 

Leaflets  narrow  ovate J.  californica 

Leaflets  ovate J.  nigra 

Leaflets  broadly  ovate J.  regia. 

Leaflets,  apex  abrupt J.  regia. 

Leaflets,  apex   gradual   taper    J.  californica 

Leaflets,  apex  attenuate J.  nigra 

Leaflets,  base  abrupt J.  nigra 

Leaflets,  base  cordate J.  californica 

Leaflets,  base  gradual  taper J.  regia. 


TRICHOMES  OF  THE  PURE  SPECIES. 

There  are  4,  or  perhaps  5  kinds  of  trichomes  in  the  pure  species  of 
Juglans  studied,  all  of  which  are  found  in  each  of  them,  which  are  trans- 
mitted kind  for  kind  to  the  hybrids,  so  that  all  types  are  to  be  found  in  the 
cross  in  the  first  and,  so  far  as  observed,  in  the  second  and  the  third  genera- 
tions also.*  The  trichomes  will  be  called  (l)  awn-shaped,  which  may  form 
groups  which  constitute  stellate  trichomes;  (2)  disk-shaped;  (3)  long 
secreting  trichomes,  apparently  of  two  kinds;  and  (4)  short  secreting 
trichomes.  All  of  the  trichomes,  except  the  awn-shaped,  are  glandular 
(figs.  4,  5,  6,  7,  and  8). 

The  trichomes  are  distributed  in  time  and  in  space  in  a  manner  which 
is  fairly  consistent  and  characteristic.  In  the  youngest  leaves  the  most 
abundant  trichomes  are  the  awn-shaped  ones.  In  old  leaves,  although  not 
plentiful,  the  short  secreting  trichomes  may  be  the  only  trichomes,  or 
nearly  the  only  ones,  to  be  found.  In  what  may  be  called  the  leaves  of 
middle  age,  all  of  the  trichomes  occur,  none  perhaps  predominating.  All 
the  glandular  trichomes  are  to  be  found  on  both  surfaces  of  the  leaves,  but 

*In  the  present  study  observations  have  been  confined  to  the  trichomes  of  young 
and  old  leaves;  only  in  one  or  two  cases  has  the  study  been  extended  to  the  stem.  No 
attempt  has  been  made  to  treat  the  general  distribution  of  trichomes  either  of  the  pure 
lines  or  of  the  hybrids. 


PURE  SPECIES  AND  HYBRIDS  OF  JUG  LANS.  19 

in  the  old  leaves  they  may  have  disappeared  from  the  upper  surface  while 
yet  some  remain  on  the  lower  (ventral)  side.  The  awn-shaped  trichomes, 
on  the  other  hand,  are  apparently  confined  to  the  ventral  surface  of  the 
leaves,  whatever  may  be  their  age.  In  this  connection  an  apparent  ad- 
aptation, which  is  a  nice  one,  is  interesting:  to  note.  In  old  leaves  the 
ventral  (or  lower)  surface  is  protected  in  various  ways  (more  especially 
presumably  by  the  position)  and  the  awn-shaped  trichomes  are  absent; 
in  the  youngest  leaves,  on  the  other  hand,  the  ventral  surface  of  the  leaves 
is  the  outer  surface  and  is  most  exposed  and  at  the  time  it  is  well  provided 
with  awn-shaped  or  stellate  trichomes. 

In  addition  to  the  variation  in  kinds  of  trichomes  which  are  present  at 
different  stages  of  development  of  the  leaf,  or  to  the  variation  in  their 
distribution  on  the  leaf,  either  of  which  have  only  been  touched  in  the 
foregoing;,  another  factor  also  must  be  taken  into  account,  namely,  the 
variation  of  the  trichomes  in  size  which  accompanies  their  distribution. 
For  example,  small  secreting  trichomes  may  be  larger  on  the  veins  than 
between  them,  and  larger  on  the  dorsal  surface  than  on  the  ventral  surface 
of  the  leaf.  Also,  the  trichomes  may  be  larger  in  one  stage  of  development 
of  the  leaf  than  in  a  subsequent  stage,  or  there  may  not  be  this  difference. 
Whatever  may  be  the  causes  of  the  variation  in  size,  the  fact  that  the  tri- 
chomes are  variable  makes  it  necessary  that  trichomes  of  analogous  stages 
of  development,  as  well  as  of  analogous  positions  on  the  leaves,  be  com- 
pared, which  has  been  done  in  all  instances.  These  circumstances,  which 
were  noted  in  the  other  hybrids,  and  were  observed  notably  in  those  of 
Oenothera,  greatly  extended  the  work,  but  at  the  same  time  quite  as  much 
enhanced  the  interest  of  the  investigation. 

JUGLANS   CALIFORNICA. 

Four  or  possibly  five  kinds  of  trichomes  are  to  be  found  on  both  surfaces 
of  the  leaves  oijuglans  calif ornica  (figs.  4  and  5).  These  are  (l)  the  awn- 
shaped  trichomes,  (2)  the  disk -shaped  trichomes,  and  (3)  the  long  and  (4) 
the  short  secreting  trichomes.  Of  the  third  type  there  may  be  two  kinds. 
The  awn-shaped  trichomes  measure  about  360  /«•  in  length;  the  disk-shaped 
trichomes  are  closely  appressed  to  the  surface;  the  long  secreting  trichomes 
are  140  p-  more  or  less  long,  and  the  short  secreting  trichomes  measure  58  p- 
more  or  less  in  length.  A  more  detailed  description  of  the  trichomes,  their 
origin  and  development,  and  their  variation,  will  be  given  in  the  following 
paragraphs. 

The  awn-shaped  trichomes  are  more  variable  than  the  other  types,  and 
since  there  were  no  apparent  causes  of  the  variation  this  type  of  trichomes 
was  not  especially  studied,  either  in  the  pure  species  or  in  the  hybrids. 
The  awn-shaped  trichomes  are  unicellular  and  frequently  are  to  be  found 
in  groups  (fig.  4,  a).  They  occur  on  the  ventral  surface  only  and  are 
especially  abundant  on  the  young  leaves, 


20 


HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 


The  disk-shaped  trichomes  are  glandular  and  are  to  be  found  on  both 
surfaces  of  the  leaves  (figf.  4, /to  n). 

The  developmental  history  of  the  trichomes  was  not  followed  in  all  par- 
ticulars. The  following:  are  some  of  the  definite  stages: 

The  trichome  originates  as  broad  and  squat  outgrowths  of  an  epidermal 
cell,  from  which  it  is  early  separated  by  a  cross-wall  that  nearly  coincides 


m 


PIG.  4.— Trichomes  of  Juglans  calif  or  nica:  a,  group  of  awn-shaped  trichomes  forming  a  stellate 
trichome,  leaf-surface  (X  84);  6  to  n,  various  stages  in  the  development  of  disk-shaped  tri- 
chomes (c,  d,  e,  h,j,  and  I X  535;  f,  g,  i,  and  m  X  860;  n  X  340);  o,  abnormal  disk-shaped  trichome 
(X535). 

with  the  adjacent  epidermal  walls.  From  its  first  appearance  as  a  papillate 
projection  the  disk-shaped  trichome  is  easily  distinguishable  from  the  be- 
ginnings of  the  other  forms.  The  young  trichome  becomes  2 -celled  by  a 
transverse  division  which  separates  the  stalk  from  the  portion  which  will 


CANNON 


PLATE  3 


Leaves  of  Juglans  regia,  from  a  tree  growing  in  Santa  Rosa,  California,  showing 
extremes  in  size  of  leaves  and  number  of  leaflets.     One-third  natural  size. 


PURE   SPECIES   AND    HYBRIDS   OF   JUGLANS.  21 

become  the  head;  all  succeeding-  divisions,  therefore,  differentiate  the  two 
regions,  namely,  stalk  and  head.  The  second  division  is  a  longitudinal 
one  in  the  terminal,  the  head-cell,  which  is  followed  by  a  transverse  divi- 
sion of  the  stalk-cell.  The  stalk  does  not  divide  further  in  normal  cells.  A 
single  monstrosity  was  observed  however  (fig.  4,  0),  in  which  the  stalk 
had  4  cells.  The  subsequent  cell-divisions  are  confined  to  the  region  of 
the  head  of  the  trichome .  The  second  and  third  divisions  of  the  head  are 
by  walls  placed  at  right  angles  to  the  preceding  division- wall,  so  that  the 
head  becomes  4  similar  cells  (fig.  4,/). 

Each  quadrant  is  next  bisected  by  a  radial  wall  (fig.  4,  £•).  After  this 
the  sequence  of  cell-division  appears  not  to  be  regular,  although  the 
octants  usually  undergo  radial  division;  and  this  process  is  repeated  until 
a  head  of  about  32  cells  is  the  result.  Whether  the  mature  trichome  may 
have  more  than  this  number  of  cells  was  not  determined,  but  in  some  in- 
stances the  presence  of  cell-walls  placed  at  right  angles  to  the  radial  walls, 
and  parallel  to  the  long  axis  of  the  trichome,  would  indicate  a  larger  num- 
ber. The  head  of  the  trichome  never  exceeds  1  cell  in  thickness.  At  an 
early  stage  in  the  development  of  the  trichome  the  peripheral  portions  of 
the  head-cells  become  somewhat  enlarged,  so  that  in  longitudinal  section 
the  edge  of  the  disk  is  bluntly  angular.  This  is  the  beginning  of  the  rim 
which  is  a  characteristic  of  the  disk-shaped  trichomes.  By  the  more  active 
growth  of  the  lower  portion  of  the  more  peripheral  cells  the  rim  is  pushed 
upwards,  i.  e.,  in  a  direction  away  from  the  surface  of  the  leaf,  until  in 
section  the  head  of  the  trichome  is  markedly  concave  (fig.  4,  ft) . 

There  is  much  variation  in  the  depth  of  the  concavity  of  the  disk,  as  in 
some  trichomes  the  tip  of  the  trichome  was  quite  flat,  while  in  others  the 
concavity  was  pronounced.  The  diameter  of  the  heads  in  the  mature  tri- 
chomes was  more  uniform,  as  the  following  measurements,  in  /*,  which 
were  made  on  representative  trichomes  selected  at  random,  would  indicate: 
117,  100,  100,  117,  100,  109. 

The  long  secreting  trichomes  are  126  /*,  more  or  less,  in  length  (fig.  5). 
The  trichome  takes  its  origin  as  a  slender  papillate  projection  of  an  epi- 
dermal cell;  it  is  early  cut  off  from  the  epidermis  by  a  transverse  wall. 
Subsequently  it  undergoes  division,  so  that  the  young  trichome  consists  of 
3  cells,  of  which  the  terminal  ones  are  the  rudiment  of  the  head  of  the  tri- 
chome. The  third  wall  is,  as  in  the  short  secreting  trichome,  almost  surely 
a  transverse  one  in  the  stalk-cell.  After  this  the  sequence  of  cell-division 
was  not  followed,  but  the  final  result  is  that  the  head  is  composed  of  4 
equal  cells,  radially  disposed,  and  the  stalk  is  composed  of  4  lineally- 
arranged  cells.  No  further  cell-divisions  occur  in  this  type  of  trichome. 
The  terminal  cells  which  constitute  the  head  become  densely  filled  with 
coarsely  granular  material;  the  cells  of  the  stalk  are  poor  in  protoplasm. 


22 


HEREDITY   AS   ILLUSTRATED   BY  TRICHOMES. 


Besides  this  type  of  long  secreting"  trichome  there  may  be  another  kind 
with  a  larger  number  of  cells  in  the  head  and  also  more  than  4  cells  in  the 
stalk.  This  form  of  trichome,  doubtful  in  Jviglans  calif ornica ,  was  often 
met  in  Juglans  nigra,  under  which  it  will  be  described,  and  in  the  hybrid 
Juglans  californica  X  Juglans  nigra.  A  few  trichomes  with  6  stalk-cells 
were  found  in  Juglans  californica,  one  of  which  is  shown  in  fig.  5,  h,  but 
none  were  seen  under  conditions  that  made  it  possible  to  surely  determine 
the  number  of  cells  in  the  head. 


> *^-LS 

n   7 


PIG.  5— Trichomes  of  Juglanx  californica:  a  to  h,  various  stages  in  the  development  of  long 
secreting  trichomes  (a,  c,  d,  f,  and  <i  535;  ft  X  1200);  i  to  o,  mature  and  young  short 
secreting  trichomes  (X  535). 

In  studying  this  type  of  trichome  it  was  found  best  to  measure  the  head 
only,  since  the  entire  trichome,  which  is  relatively  long,  varies  consider- 
ably. Wherever  possible  the  position  of  the  trichome  on  the  leaf  is  given, 
but  whenever  this  is  not  done  it  is  to  be  understood  that  in  every  instance 


PURE  SPECIES  AND  HYBRIDS  OF  JUGLANS.  23 

trichomes  from  analogous  situations  only  are  compared,  so  that  the  results 
in  such  cases  may  be  considered  just.  Following-  are  measurements,  in  p, 
on  the  length  and  diameter  of  the  heads  of  the  long-  secreting  trichomes 
which  were  taken  from  the  veins  of  the  leaves:  length,  29.4,  31.5,  35.7, 
33.6,  33.5,  31.5,  29.4,  25.2,  29.4,  29.4,  33.6,  25.2;  diameter,  25.2,  23.1, 
29.4,  25.2,  25.2,  27.3,  29.4,  31.5,  25.2,  25.2,  29.4,  27.3.  The  diameters  of 
the  heads  are  the  diameters  seriatim  of  those  whose  lengths  are  given  pre- 
viously. The  heads  average  30.6  p-  in  length  and  26. 8 /A  in  diameter.  In 
length  the  variation  from  the  average  is  13  per  cent,  and  the  variation 
from  the  average  diameter  is  15  per  cent. 

The  material  of  Juglans  calif  ornica  which  was  put  up  was  not  favorable 
for  the  study  of  the  youngest  stages  in  the  development  of  the  short  secreting 
trichomes,  so  that  a  description  of  the  mature  organ  only  is  possible  in  this 
place.  The  trichome  is  made  up  of  6  cells,  of  which  2  constitute  the  stalk 
and  4  the  head  (fig.  5,  i  to  0),  which  are  so  placed  as  to  form  a  disk  1  cell 
in  thickness. 

A  character  which  easily  distinguishes  the  short  secreting  trichomes  of 
Juglans  calif  ornica  from  those  of  Juglans  regia  or  Juglans  nigra  is  the  length 
of  the  head-cells.  As  the  figures  indicate,  the  cells  of  the  head  of  the 
short  secreting  trichome  in  Juglans  calif  ornica  are  relatively  long,  which  is 
a  consistent  character  of  the  long  secreting  trichome  as  well,  and  it  is  this 
circumstance  which  makes  advisable  the  comparison  of  the  lengths  of  the 
heads  in  both  pure  species  and  hybrids,  although  other  measurements  have 
also  been  made.  The  measurements  on  the  length  and  the  diameter  of 
the  heads,  as  well  as  the  lengths  of  the  entire  short  secreting  trichomes, 
together  with  notes  on  the  positions  occupied  by  the  trichomes,  are  pre- 
sented herewith. 

The  lengths  of  the  heads  of  the  short  secreting  trichomes  are  greater  for 
trichomes  which  are  on  the  veins  than  for  those  which  occur  between  them. 
The  measurements,  in  /*,  are:  length  of  heads,  on  veins,  35.7,  33.6,  27.3, 
29.4,  27.3,  27.4,  27.3,  33.6,  33.6,  33.6,  33.6,  29.4;  diameters  of  the  same 
heads,  respectively,  25.2,  23.1,  29.4,  25.2,  27.3,  29.4,  23.1,  31.5,  25.2,  25.2, 
29.4,  27.3;  length  of  heads,  between  veins,  27.3,  21.0,  25.2,  29.4,  25.2, 
29.4;  diameters  of  the  same  heads,  25.2,  25.2,  25.2,  25.2,  25.2,  29.4.  The 
heads  from  trichomes  on  the  veins  average  30.9  /*,  in  length  and  26.8  p  in 
diameter.  The  average  lengths  of  heads  from  between  the  veins  is  26.2  p- 
and  they  average  25.9  p-  in  diameter.  The  heads  of  trichomes  on  the  veins 
show  a  variation  in  length  of  13  per  cent  from  the  average;  those  between 
the  veins,  a  departure  of  16  per  cent  from  the  average.  The  variation 
from  the  average  in  diameter  for  the  two  series  is  respectively  13  and  8 
per  cent. 

Although  it  is  recognized  that  the  small  number  of  measurements  lessens 
the  value  of  the  results  in  any  instance,  as  a  whole  the  number  of  measure- 


24 


HEREDITY   AS   ILLUSTRATED   BY   TRICHOMES. 


ments  made  upon  analogous  organs  is  considerable,  so  that  for  a  compara- 
tive study  they  may  not  be  without  value .  It  is  therefore  of  interest  to  note 
that  the  departure  in  per  cent  from  the  average  of  trichomes  on  the  veins 
is  the  same  in  the  long  secreting  trichome  as  in  the  one  just  considered. 
Table  7  gives  the  length  of  the  trichomes,  the  length  and  diameters  of 
the  heads,  and  gives  the  area  from  which  the  trichomes  were  taken. 

TABLE  7. — Measurements  on  short  secreting  trichomes  ofjuglans  californica. 

VENTRAL  SURFACE. 


Length  of         Length 
trichome.        of  head. 

Diameter 
of  head. 

fJ.                               yu 

M 

37-7                  18.9 

(?) 

37-8                        21.  0 

25.2 

37.8                        25.2 

25.2 

39-9 

21.0 

27-3 

37-8 

18.9 

23.1 

46.2 

25.2 

25.2 

42.0 

23.  I 

29.4 

39-4 

16.8 

21  .O 

33-6 

18.9 

21  .0 

DORSAL  SURFACE. 

f- 

M 

M 

44-i 

23.1 

25.2 

54.6 

27-3 

29.4 

42.0 

25.2 

25.2 

46.2 

25.2 

27.3 

The  average  length  of  the  entire  trichome  from  the  ventral  surface  is 
39.2  f-,  from  the  dorsal  surface  is  46.7  /*.  The  average  length  of  the  heads  of 
trichomes  from  the  ventral  surface  is  21.0  p-,  the  average  diameter  is  24.6  p. 
The  averages  for  the  length  and  the  diameter  of  heads  of  trichomes  taken 
from  the  dorsal  surface  are  25. 2  p-  and  26.7  /*,  respectively. 

The  measurements  given  in  table  7  are  of  trichomes  from  old  leaves; 
those  given  just  previously  were  from  young  leaves;  we  therefore  have  an 
additional  condition  of  the  trichomes  for  comparison,  /.  e.,  a  comparison 
of  those  from  the  young  and  from  the  old  leaf.  In  the  instance  of  the 
young  leaves  the  average  length  of  the  heads  of  trichomes  which  were  on 
the  veins  was  found  to  be  greater  than  the  average  of  those  which  occur 
between  them.  In  the  old  leaves  the  heads  of  trichomes  were  larger  on 
the  dorsal  surface  than  on  the  ventral,  although  the  number  was  so  small 
that  their  relation  on  and  between  the  veins  could  not  well  be  worked  out. 
A  comparison  of  the  average  length  of  heads  from  the  ventral  surface  of 
the  old  leaf  and  of  the  young  brings  out  the  fact  that  the  heads  are  longer 


PURE   SPECIES   AND    HYBRIDS   OF   JUGLANS. 


25 


in  the  young  than  in  the  old  leaf,  which  condition  must  be  taken  into  con- 
sideration in  such  a  comparative  study  as  the  present  one. 

JUGLANS  NIGRA. 

The  following  kinds  of  trichomes  are  to  be  found  on  the  leaves  of  Juglans 
nigra:  (l)  awn-shaped,  (2)  disk-shaped,  (3)  short  secreting-  trichomes, 
and  (4)  two  types  of  the  long  secreting  trichomes  (figs.  6  and  7). 

The  awn-shaped  trichomes  measure  252  p  more  or  less  in  length  and  are 
unicellular.  They  occur  either  singly  or  in  groups  of  3  or  more.  The  awn- 
shaped  trichome  is  especially  abundant  on  the  young  leaves,  where  it  is 
found  on  the  ventral  surface  only,  although  it  persists  to  a  degree,  so  that 
scattering  trichomes  are  met  on  the  older  leaves  as  well. 

The  disk- shaped  trichomes  are  relatively  short  and  have  broadly  ex- 
panded heads  which  are  closely  appressed  to  the  surface.  In  longitudinal 
section  a  disk-shaped  trichome  is  seen  to  consist  of  a  saucer-shaped  head 
and  a  stalk  of  2  cells,  although  an  occasional  trichome  is  found  which  has 
a  stalk  of  3  cells  (fig.  6,  £).  Certain  stages  in  the  development  of  the 
disk-shaped  trichomes  were  observed.  It  was  seen  to  arise  from  the  epi- 
dermis as  a  squat  projection  which  is  soon  separated  from  it  by  a  transverse 
wall.  A  second  transverse  division  follows,  by  which  the  head  is  differ- 
entiated from  the  stalk.  The  terminal  cell  next  undergoes  division  by 
longitudinal  walls,  so  that  a  4-celled  condition  results.  The  stalk-cell,  by 
the  formation  of  a  transverse  wall,  attains  the  adult  number  of  cells,  2, 
when  cell-division  of  the  stalk  ceases. 

In  two  cases  the  failure  to  form  a  second  transverse  wall  in  the  stalk 
was  noted.  The  cells  of  the  head  divide  by  the  formation  of  longitudinally 
placed  walls  only  and  are  finally  composed  of  24  or  more  cells.  The  head, 
therefore,  is  an  expanded  plate  of  cells  1  cell  in  thickness.  The  heads  were 
75  /"•  more  or  less  in  diameter  and  the  depression  of  the  head  was  25  p-  more 
or  less  beneath  the  rim. 

Table  8  gives  in  detail  the  diameter  of  the  heads  and  the  depth  of  the 
depression  in  several  representative  and  mature  disk-shaped  trichomes. 

TABLE  8. — Measurements  of  disk  shaped  trichomes  of  Juglans  nigra. 


Diameter 

Depth  of 

Diameter 

Depth  of 

Diameter 

Depth  of 

of 

depression 

of 

depression 

of 

depression 

trichome. 

of  head. 

trichome. 

of  head. 

trichome. 

of  head. 

75^6 

29.4 

71.4 

25.2 

92.2 

21.0 

92.6 

29.4 

84.0 

25.2 

67.2 

25.2 

69.3 

25.2 

71.4 

18.9 

79.8 

25.2 

71-4 

29.4 

63.0 

21.0 

84.0 

79.8 

25.2 

84.0 

25.2 

i 

26 


HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 


In  table  8  the  figures  in  the  right-hand  columns  are  the  depths  of  the 
depressions  of  the  trichomes  whose  diameters  are  given  in  the  correspond- 
ing lines  in  the  left-hand  columns. 

The  average  diameter  of  the  heads  of  the  disk-shaped  trichomes  is  77.5  M; 
the  average  depth  of  the  depression  of  the  heads  is  25.0  /*.  Reference  to 
the  measurements  on  the  disk-shaped  trichomes  vdjuglans  calif  arnica,  given 


FIG.  6. — Trichomes  of  Juc/lann  nigra:  a,  awn  -shaped  trichome  (ca.,  242  M);  b  to  k,  development  of 
disk-shaped  trichomes  (l>,  c,  d,  e,  /,  i,  and  k,  X  ,535;  g,  h,  and  j,  X  84j;  I  to  q,  stages  in  devel- 
opment of  long  secreting  trichomes  (X  535;  I  x  1200).  (Reduced  one-fifth). 

at  page  21,  will  show  that  those  of  Juglans  nigra  are  noticeably  smaller, 
but  the  structure  and  the  development,  so  far  as  known,  of  the  two  species 
is  the  same. 

The  long  secreting  trichomes  are  of  two  types,  of  which  one  has  a  head 
of  4  cells,  and  the  other  a  head  of  about  8  cells.  Of  these  the  former  will 
be  described  first.  The  trichome  is  100  /«•  to  150  p-  in  length  (fig.  7,  a  to  e). 


PURE  SPECIES  AND  HYBRIDS  OF  JUGLANS. 


27 


It  takes  its  origin  as  a  slender  papillate  projection  of  the  epidermis,  which 
is  early  cut  off  by  a  transverse  wall,  and  which  may  occasionally  be  laid 
down  somewhat  above  the  general  level  of  the  epidermis.  From  a 
comparison  with  this  type  of  trichome  in  other  species  it  is  probable 
that  after  the  young  trichome  is  separated  from  the  epidermis  it  under- 
goes one  additional  transverse  division  when  the  distal  cell  enlarges 
and  becomes  the  head  and  the  proximal  cell  the  stalk.  Longitudinal  walls 
are  laid  down  in  the  terminal  cell  in  such  fashion  as  to  cause  the  organ- 
ization of  4  similar  and  radially  placed  cells .  With  this  the  cell  formation 
in  the  region  of  the  head  is  completed. 

The  stalk  forms  two  other  transverse  walls  (but  it  was  not  surely  de- 
termined whether  the  fourth  stalk-cell  was  formed  during  or  after  the 
divisions  in  the  head  as  given  above)  when  the  cell-divisions  in  the  region 
of  the  stalk  are  completed.  The  trichome  thus  is  composed  of  8  cells,  4  of 
which  constitute  the  head  and  4  the  stalk.  This  relation  was  fairly  con- 
stant, and  may  have  held  in  all  cases,  since  when  the  number  of  stalk-cells 
was  more  than  4  the  head  was  relatively  large  and  may  always  have  had 
more  than  4  cells. 

Table  9  gives  the  measurements  on  long  secreting  trichomes  of  the  type 
just  described. 

TABLE  !). — Measurements  on  long  secreting  trichomes  ofjuglans  nigra. 


Length 
of  head. 

Diameter 
of  head. 

Length         Diameter 
of  head.         of  head. 

Length          Diameter 
of  head.         of  head. 

/* 

21  .0 

16.8 
16.8 
16.8 
16.8 

M 
33-6 
37-8 
35-8 
42.0 
37-8 

fj.                     n 
16.8                 33.6 
16.8                 37.8 
16.8                 39.8 

21.  0                        42.O 
iS.Q                        39.9 

n                   n 
16.8                 35-7 
16.8                 33-6 
16.8                 42.0 
16.8                 33-6 
16.8                 31.5 

The  heads  of  the  trichomes  average  17.6  /"•  in  length  and  37.4  /*•  in  diam- 
eter. The  variation  from  the  average  length  is  12.5  per  cent,  and  the 
variation  from  the  average  diameter  is  14  per  cent.  From  these  measure- 
ments it  is  seen  that  the  heads  of  the  long  trichomes  of  the  4-celled  type 
are  considerably  shorter  than  the  analogous  ones  in  Juglans  calif  arnica  and 
that  the  relation  of  length  to  diameter  of  the  head  in  the  two  species  is 
also  unlike;  the  diameter  is  the  greater  \r\.  Juglans  nigra  and  the  length  is 
the  greater  in  Juglans  calif  arnica .  But  the  variation  from  the  average  length 
is  identical  in  the  two  species,  and  the  variation  from  the  average  diameter 
is  nearly  the  same. 

The  long  secreting  trichome  of  the  second  type,  that  with  more  than  4 
cells  constituting  the  head,  is  fairly  abundant  \^\J^lglans  nigra.  This  type 
of  trichome  has  a  stalk  of  from  5  to  9  cells  and  a  head  of  about  8  cells 
Only  a  few  stages  in  the  development  of  the  trichome  were  seen,  but  these 


28 


HEREDITY   AS   ILLUSTRATED   BY    TRICHOMES. 


were  sufficient  to  indicate  that  this  is  essentially  different  from  the  long 
secreting  trichome  of  the  other  type.  Fig.  7,  g,  shows  a  cross-section  of  a 
head  which  has  reached  the  5-celled  stage.  The  fifth  cell  is  formed  by 
the  laying  down  of  a  radially  placed  wall  by  which  the  mother-cell  is  divided 
unequally.  In  fig.  7,  /,  each  of  the  4  cells  is  divided  by  a  radial  wall,  so 
that  the  mature,  or  8-celled,  condition  has  resulted. 


FIG.  7. — Trichomes  of  Juglans  nigra:  a  to  e,  long  secreting  trichomes  (a,  ft,  c,  d,  and  e  X  535); 
/  to  j,  long  secreting  tricbomes  of  the  type  having  a  head  of  more  than  4  cells  ( /,  g,  and  7i, 
X535;  i  and  j  X  84);  k  to  t,  short  secreting  trichomes,  of  which  n  to  rare  from  the  region  of  the 
veins  of  the  leaves  and  *  is  from  the  leaf-margin  (k,  I,  and  o,  X  1200;  TO,  n,p,  q,  r,  *,  and  t, 
X  535).  (Reduced  one-fifth). 

In  several  trichomes  the  head  was  observed  to  be  composed  of  7  cells 
only,  but  whether  these  were  immature  or  not  was  not  determined.  From 
a  careful  study  of  this  type  of  trichome  it  would  seem  that  8  is  the  usual 
number  of  cells  in  the  head.  No  measurements  were  made  on  the  length 
of  the  heads,  but  the  following  numbers,  in  p-,  give  the  diameter  for  the 
type  of  trichome  in  question.  The  measurements  were  made  only  on 
what  were  thought  to  be  mature  forms.  Diameter  of  heads:  54.6,  50.4, 


PURE   SPECIES   AND   HYBRIDS   OF   JUGLANS. 


29 


48.3,  50.4,  50.4,  56.6,  46.2,  52.6.  That  this  is  a  type  of  trichome  distinct 
from  the  4-celled  one  is  apparent  from  the  diameters  of  the  head.  The 
smallest  head  of  the  8-celled  type  is  considerably  larger  than  the  largest  of 
the  4-celled  form ,  while  the  largest  is  more  than  25  per  cent  larger  than  these. 

No  study  was  made  of  the  excretions  or  especially  of  the  structure  of 
the  secreting  cells;  the  8-celled  type  was  the  only  one  which  gave  indica- 
tion of  marked  activity  in  this  regard.  In  such  trichomes  as  shown  in  fig. 
7 ,  c ,  it  appears  as  if  the  secretion  is  held  for  a  time  between  the  layers  of 
the  cell- walls  which  are  on  the  distal  end  of  the  trichome.  The  appear- 
ance figured  was  not  observed  in  other  trichomes. 

The  short  secreting  trichomes  occur  on  both  surfaces  of  the  leaf;  they 
measure  from  27.3  /«•  to  50.4  p-  in  length.  Whatever  may  be  the  size  of  the 
trichome  the  terminal  group  of  cells  is  relatively  short,  so  that  the  head 
has  a  squat  appearance. 

The  early  stages  in  the  development  of  the  short  secreting  trichome  are 
apparently  similar  to  those  of  the  same  type  of  trichome  in  Juglans  cali- 
fornica.  As  usual  for  the  trichomes,  the  origin  is  to  be  traced  to  rather 
narrow  outgrowths  of  epidermal  cells.  The  young  trichome  undergoes 
two  successive  transverse  divisions,  by  which  it  is  separated  from  the  epi- 
dermis and  by  which  the  initial  of  the  head  is  differentiated  from  that  of  the 
stalk.  The  following  division  is  probably  longitudinal  in  the  head-cell, 
followed  by  a  division  of  the  stalk,  by  which  the  divisions  in  the  stalk  are 
concluded,  and  this  is  followed  by  the  second  and  third  longitudinal  divisions 
of  the  head,  which  take  place  in  such  manner  that  4  radially  placed  cells 
result.  With  the  organization  of  a  head  of  4  cells  and  a  stalk  of  2  cells 
the  cell-division  in  the  trichome  ceases. 

TABLE  10.— Measurements  on  short  secreting  trichomes  from  young  leaves, 

Juglans  nigra. 


On  the  veins. 

Between  the  veins. 

Length  of  head. 

Diameterofhead. 

Length 

O-f 

Diameter 

nf 

i  . 

2. 

i  . 

2. 

I 

head. 

UI 

head. 

M 

M 

M 

P 

M- 

21  .O 

21 

0 

35-7 

31 

6 

14.7 

Not  given 

21.  0 

18 

9 

33-6 

33 

6 

16.8 

23-3 

18 

9 

33-6 

29 

4 

16.8 

16.8 

29.4 

14.7 

16.8 

33-6 

14-7 

21  .O 

35-7 

16.8 

16.8 

3x-4 

16.8 

29.4 

16.8 

16.8 

25.2 

29-4 

16.8 

21  .O 

33-6 

21.  0 

35-7 

16.8 

29-4 

21  .O 

31.6 

30 


HEREDITY    AS    ILLUSTRATED    BY    TRICHOMES. 


On  the  veins  the  average  length  of  the  heads  is  20.8  A*;  the  average 
diameter  is  31.5  p-.  Between  the  veins  the  average  length  of  the  heads  is 
16.2  A4;  the  diameters  are  not  given.  The  variation  from  the  average 
length  of  the  heads  of  trichomes  from  the  region  of  the  veins  is  31.5  per 
cent;  the  variation  in  diameter  is  60  per  cent.  The  variation  in  length  of 
heads  from  trichomes  which  were  from  between  the  veins  is  0.9  per  cent. 

TABLE  11. — Measurements  of short  secreting  trichomes  from  old  leaves, 
Jiiglans  nigra. 


Dorsal  surface. 

Ventral  surface. 

Length  of 

Length  of 

Diameter 

Length  of 

Length  of 

Diameter 

trichome. 

head.           of  head. 

trichome. 

head. 

of  head. 

M 

M 

M 

M 

M 

M 

27-3 

I4./                      25.2 

27-3 

14.7 

27-3 

27-3 

12.6                 25.2 

23.1 

12.6 

21  .O 

*50-4 

23-0           31-5 

25.2 

12.6 

25.2 

37-8 

1                  21.0                        33.6                         25.2 

12.6 

25.2 

33.6 

16.8                  27.3                  25.2 

12.6 

21  .O 

33-6 

12.6 

21  .O 

*3i-5 

*2I.O 

*33-6 

25.2 

12.6 

21.  0 

27-3 

12.6 

12.6 

i 

25.2 

11.5 

18.9 

i 

*Vein. 

The  average  length  of  heads  from  the  dorsal  surface  is  16.8  A*;  the  aver- 
age length  of  heads  of  trichomes  from  the  ventral  surface,  including  the 
abnormally  large  one,  is  13.4  A*;  omitting  that  one,  the  length  is  12.5  A*. 
The  average  length  of  the  trichomes  on  the  dorsal  surface  is  35.0  A4;  the 
average  length  on  the  ventral  surface  is  26.1  A4-  The  variation  in  length 
of  the  heads  from  the  dorsal  surface  is  20  per  cent;  from  the  ventral  sur- 
face 62  per  cent. 

From  the  study  of  the  short  secreting  trichome  it  appears,  therefore, 
that  trichomes  from  the  regions  of  the  veins  in  the  young  leaves  have 
longer  heads  than  those  from  the  areas  between  them,  and  that  the  length 
of  the  heads  of  trichomes  from  the  dorsal  surface  of  old  leaves  is  greater 
than  from  the  ventral  surface.  The  entire  trichomes  also  are  longer  on 
the  dorsal  surface;  and  the  heads  of  trichomes  of  young  leaves  are  greater 
than  the  length  in  old  leaves. 

JUGLANS   REGIA. 

The  types  of  trichomes  which  were  seen  in  the  two  species  of  Jug  Ian  s  as 
above  described  were  observed  in  Jug  lam  regia  also.  These  are,  (l)  the 
awn-shaped  trichome;  (2)  the  disk-shaped  trichome;  (3)  one  or  perhaps 
two  types  of  long  secreting  trichomes,  and  (4)  the  short  secreting  tri- 
chomes (fig.  8). 


PURE  SPECIES  AND  HYBRIDS  OF  JUGLANS.  31 

The  awn-shaped  trichomes  occur  on  the  ventral  surface  of  the  leaves 
only  and  are  especially  abundant  on  young'  leaves.  They  sometimes  are 
found  in  groups,  especially  in  young"  leaves,  but  in  older  ones  they  usually 
occur  singly;  they  measure  126  /*,  more  or  less,  in  length. 

As  distinguished  from  the  awn-shaped  trichome,  which  is  unicellular 
and  non-secreting,  the  other  forms  are  glandular  and  are  multicellular. 
The  mature  disk-shaped  trichome  consists  of  a  2-celled  stalk  and  a  flat- 
tened head,  1  cell  in  thickness,  of  about  32  cells.  The  youngest  stages  in 
development  were  not  seen.  After  the  head  has  been  differentiated  and 
divided  into  4  similar  cells,  each  quadrant  becomes  divided  unequally  by 
radial  walls,  so  that  a  head  of  8  cells  results  (fig.  8,  c).  Later  each  octant 
is  divided  by  walls  which  extend  outwards  from  the  newly-formed  octant 
walls  and  a  head  of  16  cells  is  the  result.  The  actual  formation  of  walls 
subsequently  was  not  observed.  Usually  the  cell-division  in  the  head 
proceeds  in  a  regular  manner,  so  that  the  product  is  symmetrical,  but  a 
single  monstrous  trichome  was  seen  in  which  this  had  not  been  the  case. 
The  history  of  this  trichome,  naturally,  could  not  be  studied;  it  is  shown 
at  i,  fig.  8. 

Measurements  on  the  disk-shaped  trichomes  show  that  they  vary  both 
in  diameter  and  in  depth  in  a  regular  manner,  which  is  to  be  related  to  the 
relative  age  of  the  leaf  where  found.  Following  are  given  measurements, 
in  /*,  on  the  diameter  and  the  depth  of  disk-shaped  trichomes  on  young 
leaves:  diameter,  105.0,  109.2,  105.0;  depth,  8.4,  8.4,  12.6,  16.8,  respec- 
tively. The  trichomes  are  not  so  abundant  on  old  leaves  and,  as  the  fol- 
lowing fignres  (/*)  show,  they  are  also  of  less  diameter;  the  depth  is  not 
given:  diameter,  58.8,  50.4,  46.2,  46.2,  58.8,  46.6,  42.0,  42.0,  50.4,  58.8, 
averaging  45.0. 

The  long  secreting  trichome  measures  96  /*,  more  or  less,  in  length. 
The  mature  trichome  consists  of  a  head  of  4  cells  and  a  stalk  of  4  cells 
(fig.  8,  j  to  m).  It  takes  its  origin  as  a  papillate  projection  of  an  epider- 
mal cell  which  is  early  cut  off  from  the  epidermis  by  a  wall  parallel  to  it. 
The  young  trichome  next  undergoes  transverse  division,  by  which  the  por- 
tion which  is  to  become  the  head  is  differentiated  from  the  portion  which  is 
to  be  the  stalk.  The  second  division  and  the  third  division  were  not  seen, 
but  are  probably  longitudinal  in  the  head  rudiment  and  transverse  in  the 
stalk  rudiments,  respectively,  as  in  the  long-  secreting  trichomes  of  the  pure 
species,  as  well  as  hybrids,  where  the  sequence  has  been  carefully  followed. 

Either  following  the  divisions  by  which  the  head  becomes  4-celled,  or 
part  passu  with  these,  the  two  final  divisions  of  the  stalk  take  place.  Of 
these  divisions  that  of  the  outer  cell  occurs  first.  When  the  trichome  is 
mature  the  head  is  relatively  long,  but  the  material  at  hand  was  not  favor- 
able for  a  comparative  study  of  the  long  secreting  trichome,  so  that  a  more 
precise  statement  can  not  at  present  be  made.  It  was  almost  entirely 
absent  from  old  leaves  and  not  very  abundant  on  the  young  ones.  The 


32 


HEREDITY    AS    ILLUSTRATED    BY    TRICHOMES. 


FIG.  8— Tricbomes  of  Juglans  regia:  a,  awn-shaped  trichomes  (X  84):  6  to  h,  stages  in  the  de- 
velopment of  disk-shaped  trichomesj  (>;  535);  /,  abnormal  disk-shaped  trichome  (  X  1200); 
./  to^w,  long  secreting  trichomes  (./  and  k  X  1200;  I  and  in  \  536):  n  to  r,  development  of 
short  secreting  trichomes,  of  which  r  is  a  transverse  view  of  q  (n,  o,  and  p  X  1200 ;  q  and 
»'X535);  *  to  v,  short  secreting  trichomes  from  rather  old  leaf  (X  535);  w,  short  secreting 
trichomes  from  growing  tip  of  stem  (x  535).  (All  figures  reduced  one-fourth.) 


PURE   SPECIES   AND   HYBRIDS   OF   JUG  LANS. 


33 


second  form  of  long'  secreting"  trichome ,  which  was  found  in  Juglans  nigra 
especially,  was  not  surely  determined  for  this  species;  if ,  indeed,  it  occurs 
in  Juglans  regia  at  all  it  is  rare. 

The  short  secreting-  trichome  is  to  be  found  on  both  surfaces  of  the 
leaves,  both  old  and  young-,  althoug'h  it  is  more  abundant  on  the  latter. 
Only  two  of  the  young-  stages  in  development  of  the  short  trichome  were 
observed.  After  separation  from  the  epidermis  the  next  succeeding-  cell- 
division  cuts  of  the  head-cell  from  the  stalk-cell.  When  mature  the  tri- 
chome consists  of  2  stalk-cells  and  4  head-cells  (shown  in  figf.  8,  n  to  w). 

The  short  secreting-  trichome  shows  a  considerable  variation  in  size  which 
appears  to  be  directly  associated  with  the  position  occupied  by  it.  The 
variation  is  especially  noticeable  in  the  length  of  the  head.  Table  12 
illustrates  the  rang-e  of  variation  in  length  of  head  in  young-  and  old  leaves 
and  in  stem. 

TABLE  12. — Measurements  of  short  secreting  trichome j, 
Juglans  regia. — Length  of  head. 


Young 
leaf. 

Young 
stem. 

Old 
leaf. 

29.4 

21.  0 

16.8 

27.4 

21  .O 

8.4 

29.4 

29.4 

16.8 

29.4 

29.4 

16.8 

29.4 

12.6 

25.2 

25.2 

The  average  length  of  the  trichomes  from  the  dorsal  surface  is  29.2  /*, 
from  the  ventral  surface,  25.2  p.  The  length  of  the  heads  of  dorsally 
placed  trichomes  is  also  slightly  longer  than  those  of  the  ventrally  placed 
ones;  those  of  the  former  average  14.3  p-  and  those  of  the  latter  is  13.6  /* 
(table  13). 

TABLK  13. — Measurements  on  short  secreting  trichomes,  Juglans  regia. 


Dorsal  surface. 

Ventral  surface. 

Length  of 
trichome. 

Length  of 
head. 

Diameter 
of  head. 

Length  of 
trichome. 

Length  of 
head. 

Diameter 
of  head. 

/" 
28.4 
23.1 
33-6 
29.4 

3i-5 
29.4 

M 
16.8 
14.7 
14.7 

12.6 

14.7 

12.6 

M 

27-3 
25.2 
23.1 
23.0 
25.2 
23.1 

M 
28  4 
25.2 
25.2 
25.2 
23.1 
25.2 

M 
14.7 
12.6 
12.6 
12.6 

14-7 

14.7 

M 
21.  0 
21  .O 
21.  0 

25.2 
25.2 
21.0 

34  HEREDITY   AS   ILLUSTRATED   BY   TRICHOMES. 

GENERAL  COMPARISON  OF  THE  LEAVES  AND  THE  TRICHOMES 
IN  PURE  SPECIES  OF  JUGLANS. 

In  leaf-characters  the  pure  species  of  Juglans  are  fairly  sharply  separated 
from  one  another.  For  example,  in  number  of  leaflets  regia  has  less  than 
one-half  those  of  either  of  the  other  species,  and  the  range  in  number  of 
leaflets  in  calif ornica  is  considerably  less  than  the  range  in  nigra.  In  form 
of  the  leaflets  the  species  are  not  so  sharply  distinguishable.  The  leaf- 
character,  however,  is  a  good  one  to  observe  in  the  hybrids  between  these 
species. 

Although  somewhat  difficult  in  all  cases  to  speak  with  certainty,  with  a 
single  exception  it  appears  that  all  of  the  types  of  trichomes  occurring  in 
any  one  species  may  be  found  in  all,  and,  further,  that  the  leading  points 
in  structure  and  development  are  the  same  wherever  the  trichome  is  to  be 
found.  Four  types  of  trichomes  occur  in  calif  ornica  and  regia  and  an  ad- 
ditional type  in  nigra.  Of  these,  one  form  is  unicellular  and  lifeless,  the 
others  are  multicellular  and  glandular.  So  far  as  could  be  determined 
from  the  material  at  hand,  each  type  of  multicellular  trichome  appears  to 
have  its  peculiar  manner  of  development,  as  suggested  above,  to  which  it 
adheres  with  great  consistency.  After  the  trichome  rudiment  has  been 
formed  the  divisions  of  the  short  secreting  trichome  were,  first,  a  cross- 
wall  separating  the  head  from  the  stalk;  second,  a  longitudinal  wall  in  the 
head;  third,  a  cross- wall  in  the  stalk;  and  fourth  and  fifth,  two  longitudinal 
divisions  of  the  head.  The  long  secreting  trichome  experiences  the  same 
sequence  of  cell-divisions  and  to  them  adds  2  in  the  stalk.  The  sequence 
of  division  of  the  disk-shaped  trichome  is  the  same  for  the  first  two  divisions, 
after  which  it  divides  in  another  manner,  which  is  thought  to  be,  although 
not  actually  proved  to  be,  also  consistent.  As  a  result  of  the  divisions  the 
heads  of  the  various  trichomes  become  4,  8,  and  probably  32  celled. 

Although  abnormalities  were  observed  they  were  not  always  intermediate 
between  any  of  the  types,  and  in  fact  no  intermediates  were  noticed. 

The  multicellular  trichomes  varied  in  size  to  a  considerable  degree,  but 
the  variation  was  fairly  consistent  and  was  usually  to  be  related  to  position 
on  the  leaf  or  to  age  of  the  leaf.  The  trichomes  and  the  heads  of  trichomes 
also  were  each  longer  on  the  dorsal  surface  than  on  the  ventral,  on  old 
than  on  young  leaves,  and  on  the  veins  than  between  them.  Nothing  was 
observed  on  the  changes  in  size  of  the  trichome-heads  accompanying  their 
functional  activity,  if  such  occurs,  and  the  variation  is  probably  to  be  ex- 
plained on  other  grounds. 

The  heads  of  the  trichomes  in  the  pure  species  Juglans  californica  are 
markedly  longer  than  those  of  either  of  the  other  species;  those  of  Juglans 
nigra  are  shortest.  The  differences  in  length  lie  from  50  to  75  per  cent, 
so  that  this  is  a  very  good  character  to  study  in  any  cross  in  which  Juglans 
californica  is  one  of  the  parents. 


HYBRIDS  OF  JUGLANS.  35 

HYBRIDS  OF  JUGLANS. 

LEAVES  OF  THE  HYBRIDS. 
JUGLANS  CALIFORNICA  x  JUGLANS  NIGRA. 

The  walnut  hybrid  of  this  parentage  was  formed  by  Mr.  Burbank  in 
1878  and  is  known  as  the  "Royal."  One  specimen  of  the  first  generation, 
which  is  the  sole  representative  of  this  generation,  is  now  growing  at  the 
Sebastopol  ranch  of  Mr.  Burbank.  It  is  a  very  handsome  tree,  about  20 
meters  high,  with  a  wide-spreading  top.  Some  second  and  some  third 
generation  seedlings,  the  latter  known  as  the  "Beeson"  walnut,  are  grow- 
ing near  the  Royal  at  Sebastopol. 

The  Royal  is  distinguished  by  its  rapid  growth,  by  the  fine  grain  of  the 
wood,  and  by  the  relatively  wide  annual  rings  of  growth.  The  second 
and  third  generation  seedlings  are  extremely  variable.  This  is  noted  in 
size,  in  vigor  of  the  plants,  and  in  the  color,  size,  texture,  and  other  char- 
acters of  the  leaves.  The  Royal  fruits  abundantly;  the  fruiting  character 
of  the  later  generations  is  not  known. 

The  leaves  are  composed  of  from  5  to  9  or  more  pairs  of  leaflets,  which 
may  be  strictly  opposite  on  the  rachis  or  may  be  more  or  less  in  alternation 
(plate  4).  The  terminal  leaflet  is  usually  smaller  than  the  other  leaflets, 
although  it  may  also  be  larger  than  they  are.  It  is  generally  single,  but 
occasionally  a  doubling  is  to  be  seen.  In  form  the  leaflets  vary  from  lan- 
ceolate, with  a  somewhat  attentuate  apex,  to  broadly  ovate  with  an  apex 
which  is  acuminate.  The  bases  may  be  either  abrupt  or  even  cordate.  The 
leaflets  are  serrate  and  exhibit  all  grades  between  fine  and  coarse  serration . 
The  surface  of  the  leaves  is  either  smooth  or  roughened,  the  veins  of  the 
dorsal  surface  may  be  either  sunken  or  may  be  raised — generally  the  latter. 
As  a  whole,  any  type  of  leaflet,  or  quality  appertaining  to  a  leaflet,  holds 
throughout  all  the  other  leaflets  of  the  leaf. 

So  far  as  the  general  observations  on  the  leaves  of  the  three  generations 
of  the  walnut  hybrid  go,  the  leaves  show  some  grade  of  intermediacy.  No 
leaf  was  seen  which  was  not  distinguishable  from  the  leaves  of  either  of 
the  pure  race,  and  all  of  the  leaves  examined  showed  characters  of  both. 
How  far  this  remark  will  be  good  when  each  second  or  third  generation 
plant  is  studied  separately  is  not  known. 

JUGLANS  CALIFORNICA  X  JUGLANS  REGIA. 

This  walnut  hybrid,  known  as  the  "Paradox,"  originated  from  a  cross 
made  by  Mr.  Burbank  in  1887.  Four  trees  of  the  first  generation  are  at 
present  growing  in  the  street  in  front  of  the  experimental  tract  of  Mr. 
Burbank  in  Santa  Rosa.  Seedlings  of  the  second  (and  third)  generation 
are  at  the  Sebastopol  ranch.  The  first-generation  trees  are  about  28 
meters  high  and  about  25  meters  spread.  The  bole  of  one  of  these  trees, 
at  a  point  1.5  meters  above  the  surface  of  the  ground,  measured  30  cm.  in 


36 


HEREDITY   AS   ILLUSTRATED   BY   TRICHOMES. 


diameter.  Like  the  other  walnut  hybrid  (the  Royal)  this  is  a  rapid  grower; 
the  wood  is  fine-grained,  and  the  annual  rings  are  relatively  wide.  The 
Paradox,  unlike  the  Royal,  however,  bears  little  fruit. 

The  leaves  of  the  first  generation  are  extremely  variable  in  mimber  of 
leaflets,  in  size,  and  in  many  other  particulars.  Leaves  with  3,  4,  5,  6,  and 
7  pairs  of  leaflets  were  selected  for  photographing  (plate  7). 

The  number  of  pairs  of  leaflets  are  said  by  Mr.  Burbank  to  reach  as 
great  a  number  as  19,  though  I  did  not  see  leaves  with  so  great  a  number. 
Measurements  on  the  extreme  variation  of  the  leaves  were  not  made,  but 
observation  would  indicate  that  the  range  may  easily  exceed  500  per  cent. 

The  leaflets  vary  in  outline  from  ovate  to  broadly  ovate.  The  apices 
are  acuminate  or  abrupt;  sometimes  they  taper  much  as  in  the  pure  species 
Juglans  calif ornica.  The  bases  are  abrupt  or  even  cordate;  none  are  broadly 
cuneate,  as  in  pure  species  referred  to.  The  leaf -margins  are  remotely 
serrate.  In  all  specimens  examined  the  veins  of  the  dorsal  surface  are 
prominent.  The  leaf-surfaces  are  usually  smooth,  although  they  may  also 
be  roughened.  Thus  each  leaflet  shows  characters  of  both  pure  lines. 
Whether  a  similar  condition  will  be  seen  to  obtain  in  the  later  generations 
can  not  at  present  be  stated. 

The  great  tendency  of  the  leaves  of  the  Paradox  hybrid  to  vary  has  been 
remarked  by  Mr.  Burbank,  who  tells  me  that  in  company  of  two  eminent 
scientists  he  selected  400  leaves  or  leaflets  easily  recognizable  as  different 
from  one  another. 

TABLE  14. — Leaves  of  Juglans  calif  ornica  X  Juglans  nigra%Fv  compared  with 
those  of  the  pure  species. 


J.  nigra. 

J.  californica  X  nigra. 

J.  californica. 

Leaves  with  13  to  21  leaf- 
lets 
Leaflets  ovate  

Leaves  with  n  to  19  leaf- 
lets. 
Leaflets  lanceolate  to 

Leaves   with    19    to   21 
leaflets. 
Leaflets  narrowly  ovate 

Leaflets,  apex  attenuate  .... 
Leaflets,  base  abrupt  

broadly  ovate. 
Leaflets,  apex  acuminate 

Leaflets,  base  abrupt   or 
cordate. 

Leaflets,  apex   tapers 
gradually. 
Leaflets,  base  cordate. 

TABLE  15. — Leaves  of  Juglans  californica  X  Juglans  regia,  F±,  compared  with 
those  of  the  pure  species. 


J.  regia. 

J.  californica  X  regia. 

J.  californica. 

Leaves  with   5  to  7  leaf- 
lets. 
Leaflets  broadly  ovate  

Leaflets,  apex  abrupt  

Leaflets,  base  gradually 
tapering. 

Leaves  with  7  to  15  leaf- 
lets. 
Leaflets  ovate  to  broadly 
ovate. 
Leaflets,  apex  abrupt,  or 
acuminate. 
Leaflets,  base  abrupt,  or 
cordate. 

Leaves    with    19    to    21 
leaflets. 
Leaflets  narrowly  ovate. 

Leaflets,     apex     tapers 
gradually. 
Leaflets,  base  cordate. 

CANNON 


PLATE  4 


Leaves  of  Juglans  californica  X  Juglans  nigra,  FI,  from  Luther  Burbank's  Sebastopol  ranch,  to 
illustrate  the  range  in  variation.     One-third  natural  size. 


CANNON 


Leaves  of  Juglans  californica  X  Juglans  nigra,  p2,  to  show  extremes  in  variation  of  leaves  and 
leaflets.  From  Luther  Burbank's  Sebastopol,  California,  ranch.  The  figures  are  reduced 
equally.  One-third  natural  size. 


HYBRIDS  OF  JUGLANS.  37 

TRICHOMES  OF  JUGLANS  CALIFORNICA  X  JUGLANS  NIGRA,  Ft. 

Four  types  of  trichomes  occur  on  the  leaves  of  this  hybrid.  These  are 
(1)  the  awn-shaped  trichome,  (2)  the  disk-shaped  trichome,  (3)  the  long- 
secreting-  trichome  (of  two  sorts),  and  (4)  the  short  secreting  trichome. 
The  awn-shaped  trichomes  are  unicellular  and  usually  occur  singly;  they 
are  found  upon  the  ventral  surface  only  of  the  leaves  and  measure  211  /* 
more  or  less  in  length.  While  this  type  of  trichome  is  especially  abundant 
in  young  leaves,  it  persists  to  a  degree,  so  that  it  may  be  found  in  mature 
leaves  as  well. 

As  in  the  pure  species,  the  disk-shaped  trichomes  are  composed  of  a 
short  stalk  and  a  broadly  expanded  head,  of  which  the  center  is  depressed 
(fig.  9).  The  supporting  stalk  is  2-celled;  the  head  is  of  many,  perhaps 
always  of  32  cells,  but  is  only  1  cell  in  thickness.  The  origin  and  develop- 
ment of  the  trichome  were  not  shown  satisfactorily  in  the  material  at  hand, 
so  that  a  description  of  these  processes  in  hybrids  of  the  first  generation 
can  not  be  given.  Measurements,  in  /*,  on  the  diameter  of  the  trichome 
and  on  the  depth  of  the  depression  of  the  head  were  made  as  follows: 
diameter,  107.1,  109.2,  105.0,  100.8,  92.4,  92.2;  depth,  8.4,  8.4,  18.9, 
16.8,  21.0,  12.6,  respectively. 

The  longer  secreting  trichomes  measure  126  p  more  or  less  in  length 
and  are  of  two  types,  which  appear  to  be  distinct.  The  more  usual  form 
consists  of  a  stalk  of  4  cells  and  a  head  of  4  cells  which  are  radiate.  The 
other  form  is  composed  of  a  stalk  of  a  varying  number  of  cells,  usually 
more  than  4,  and  a  flat  expanded  head  of  about  8  cells.  Although  both 
forms  occur  on  both  surfaces  of  the  same  leaf,  the  latter  is  to  be  found 
chiefly,  perhaps,  on  the  veins.  The  more  common  form  will  be  described 
first.  After  the  young  trichome  has  been  cut  off  from  the  epidermis  a 
transverse  wall  appears  which  separates  the  head  portion  from  the  part 
which  will  become  the  stalk.  The  next  division  of  the  trichome  is  also  a 
transverse  one  and  occurs  in  the  stalk-cell.  The  third  division  takes  place 
in  the  head-cell  and  is  a  longitudinal  one.  What  the  order  of  cell-divisions 
was  after  this  was  not  learned.  The  heads  of  the  mature  trichomes  are 
composed  of  rather  long  cells,  so  that  in  effect  the  head  is  neither  like  the 
head  of  the  long  secreting  trichome  in  Juglans  nigra,  nor  like  that  in 
Juglans  calif ornica,  but  holds  an  intermediate  position.  The  following 
measurements,  in  /*,  were  made  on  the  heads  of  mature  trichomes:  length, 
21.0,  25.2,  25.2,  21.0,  25.2,  21.0,  33.6,23.1,  21.0,  25.2,  23.1,  25.2.  The 
average  of  these  is  24.1  p. 

The  early  stages  in  the  development  of  the  second  type  of  long  secreting 
trichome,  that  with  a  head  of  8  cells,  were  for  the  most  part  not  seen,  but 
certain  peculiarities  in  the  cell -divisions  of  the  head  may  be  recorded.  In 
transverse  sections  of  the  heads  of  all  of  the  other  trichomes  studied  the 


38 


HEREDITY   AS    ILLUSTRATED    BY   TRICHOMES. 


FIG.  9. — Trichomes  ofjuglans  californica  <  nigra,  Fj:  a,  awn-shaped  trichome  from  a  leaf;  6  to  d, 
disk-shaped  trichomes  ;  e  to  i,  some  stages  in  the  development  of  long  secreting  trichomes ; 
j,  transverse  section  of  head  of  mature  long  secreting  trichome ;  k  to  m,  long  secreting  tri- 
chomes ;  n  to  r,  long  secreting  trichomes  of  the  type  having  a  head  of  more  than  4  cells; 
n,  longitudinal  sections  of  the  head  of  a  single  trichome;  o  to  q,  views  of  long  secreting  tri- 
chomes ;  r,  transverse  sections  of  head  of  long  secreting  trichomes  of  different  ages ;  *,  some 
stages  in-the  development  of  short  secreting  trichomes.  (All  figures  X  585;  reduced  one- 
fourth.) 


HYBRIDS  OF  JUG  LANS. 


39 


first  three  cell-walls  are  so  arranged  that  a  + -shaped  figure  results.  In 
this  type  of  the  long-  secreting  trichome,  however,  in  which  the  4,  5,  and 
6  cell  stages  of  the  head  were  seen,  the  arrangement  of  the  walls  is  quite 
different  and  indicates  that  there  may  have  been  a  very  different  sequence 
in  the  laying  down  of  the  walls  from  what  ordinarily  occurs.  As  nuclear 
division  was  not  seen  in  the  heads,  however,  what  the  sequence  of  cell- 
division  was  can  not  at  present  be  told.  In  length  of  heads  this  type  of 
long  secreting  trichome  is  not  different  from  the  type  having  a  head  of  4 
cells.  The  following  measurements,  in  p,  were  made:  length,  21.0,  21.0, 
25.2,  29.2.  The  diameter  of  such  heads,  however,  is  usually  considerably 
greater. 

The  short  secreting  trichomes  were  found  on  both  surfaces  of  all  leaves 
but  the  oldest  where  they  were  apparently  absent  from  the  ventral  side. 
In  length  they  range  from  35.7  /*  to  54.6  /*  and  the  size  is  evidently  to  be 
associated  either  with  position  on  the  leaf  or  with  its  age.  The  heads  also 
of  the  trichomes  in  both  pure  species  vary  greatly  in  length,  and  here, 
also,  the  variation  is  probably  in  some  manner  connected  with  the  position 
occupied  on  the  leaf,  or  with  the  condition  of  the  leaf,  but  when  taken  into 
consideration  does  not  vitiate  a  comparison  of  similar  organs  which  occur 
under  analogous  conditions.  The  material  studied  did  not  permit  an  obser- 
vation of  the  development  of  this  type  of  trichome,  so  that  for  the  present 
a  description  of  it  must  be  omitted.  The  mature  trichome  consists  of  a 
2 -celled  stalk  and  a  head  of  4  cells  placed  radially.  The  first  division  of 
the  young  trichome  probably  separates  the  stalk  region  from  the  head 
region,  as  in  both  the  pure  lines,  and  the  divisions  of  the  head  are  longi- 
tudinal. Measurements  on  the  short  secreting  trichomes  are  given  in 
table  16. 

TABLE  IB. — Measurements  on  the  short  secreting  trichomes  of  Juglans  calif ornica 

X  Juglans  nigra,  F^. 


Trichomes  from  veins 

Trichomes  from  dorsal  sur- 

of young  leaves. 

face 

of  old  leaves.  t 

Length       Diameter 
of  heads.*    of  heads. 

Length  of 
trichomes. 

Length 
of  heads. 

Diameter 
of  heads. 

M 

M 

^ 

f- 

M 

21  .0 

21.  0 

37-8 

16.8 

25.2 

16.8 

25.2 

35-7 

18.9 

25.2 

21  .0 

25.2 

39-9 

18.9 

23.1 

21  .O 

25.2 

42.0 

21  .O 

25.2 

18.9 

25.2 

42.0 

23-1 

25.2 

21  .O 

27-3 

J54-6 

25.2                   29.4 

16.8 

25.2 

21.0 

25.2 

21  .O 

25.2 

*The  average  length  of  these  heads  is  19.8  /u;  the  average  in  diameter  is  25.1 
tThis  type  of  trichome  is  absent  from  the  ventral  surface  of  old  leaves. 
tVein. 


40 


HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 


The  average  length  of  the  heads  of  trichomes  of  old  leaves  is  20.6  /*; 
the  average  diameter  is  25.5  p-.  Thus  the  lengths  of  heads  in  the  old  leaf 
and  in  the  young  leaf  are  practically  the  same.  In  the  pure  species,  where 
the  comparison  is  made,  the  young  leaves  were  found  to  have  the  largest 
trichomes.  It  is  possible  that  a  comparison  of  trichomes  from  analogous 
leaf-surfaces  would  in  this  instance  also  give  similar  results. 

TRICHOMES  OF  JUGLANS  CALIFORNICA  X  JUGLANS  REGIA,  Fx. 

As  in  the  pure  species  Juglans  calif ornica  and  fuglans  regia,  so  also  in 
the  hybrid,  4  types  of  trichomes  are  to  be  distinguished,  namely,  the  awn- 
shaped,  the  disk-shaped,  and  the  long  and  the  short  secreting  trichomes. 
The  unusual  type  of  long  secreting  trichome  which  was  observed  in  the 
other  hybrid  Juglans  and  in  the  pure  species  Juglans  nigra  appears  not 
to  be  present  in  this  hybrid  (fig.  10). 

TABLE  17. — Measurements  on  head?,  of  long  secreting  trichomes  oj 
Juglans  californica  X  Juglans  regia,  F^. 


Length. 

Diameter. 

Length. 

Diameter. 

M 

M 

M 

M 

21  .O 

37-8 

25.2 

37-8 

25.2 

42.0 

23.1 

50.4 

21  .0 

46  .2 

23.1 

39-9 

21.0 

33-6 

21.0 

37-8 

The  awn-shaped  trichomes  are  most  abundant  in  the  young  leaves,  where 
they  occur  on  the  ventral  surface  only.  They  are  of  the  usual  type,  uni- 
cellular, and  range  around  211.6  /"•  in  length. 

The  disk- shaped  trichomes,  which  in  structure  are  quite  like  those  in 
both  pure  lines,  consist  of  a  flattened,  multicellular  head  borne  on  a  stalk 
of  2  cells.  In  young  leaves  the  trichomes  are  to  be  found  on  both  sur- 
faces; in  older  ones  they  occur  on  the  ventral  surface  only.  Mature  tri- 
chomes of  this  type  vary  gTeatly  in  diameter,  but,  so  far  as  observed,  the 
differences  in  diameter  are  more  or  less  closely  associated  with  differences 
in  position  on  the  leaves,  and  possibly  also  with  the  age  of  the  leaf.  The 
following  measurements,  in  /*,  give  the  diameters  of  representative  disk- 
shaped  trichomes:  71.4,.  73.6,  67.2,  65.0,  75.6,92.2,79.8.  Of  these  the 
two  last  were  from  the  region  of  the  veins.  The  average  diameter  of  the 
heads  of  the  trichomes  is  74.9  p-.  A  few  measurements  were  made  also  to 
trace  an  additional  possible  character— the  depth  of  the  depression— for 
comparison  with  the  pure  lines.  These  measurements  are  as  follows,  in  p-: 
diameter,  63.0,  84.0,  92.4,  79.8;  depth,  12.6,  16.8,  12.6,  12.6,  respectively. 

The  long  secreting  trichomes  are  most  abundant  on  young  leaves  and 
are  more  abundant  on  the  veins  than  between  them.  They  occur  on  both 
leaf -surf  aces.  The  trichome  studied  consists  of  a  head  with  4  radiate 
cells  and  a  stalk  of  4  cells.  None  of  the  type  with  a  head  of  8  cells  was 


CANNON 


PLATE  6 


Leaves  of  Juglans  californica  X  Juglans  nigra,  ¥3,  to  show  the  variation  of  the  leaves  and  the 
leaflets  in  size  and  in  other  qualities.  From  Luther  Burbank's  Sebastopol  ranch.  Two- 
fifths  natural  size. 


CANNON 


Leaves  of  Juglans  califormca  X  Juglans  regia,  FI,  from  Santa  Rosa,  California.  The  "royal" 
walnut  of  Luther  Burbank.  The  figures  of  this  plate,  and  of  the  following  one,  illustrate 
the  range  of  variation  in  size  of  leaves  and  in  the  number,  size,  and  other  qualities  of  the 
leaflets.  One-third  natural  size. 


CANNON 


PLATE  8 


Leaves  of  Juglans  calif ornica  X  Juglans  regia,  FI,  from  Luther  Burbank's  nursery,  Santa  Rosa, 
California,  showing  the  range  of  the  variation  of  leaves  and  leaflets.  Compare  with  the 
preceding  plate.  One-third  natural  size. 


HYBRIDS   OF   JUGLANS. 


41 


seen.  Although  the  development  of  this  trichome  was  not  studied  par- 
ticularly, the  material  examined  indicated  that  the  sequence  of  cell-divisions 
may  be  the  same  as  in  the  same  type  of  trichome  in  both  pure  lines,  and 
thus  it  conforms  with  the  sequence  in  cell-divisions  in  all  of  the  other  types 
of  multicellular  trichomes  in  Juglans.  Table  17  presents  the  measure- 
ments made  on  the  heads  of  this  form  of  trichome.  The  average  length 
of  the  heads  is  22.5  P-;  diameter,  40.6 


FIG.  10.— Trichomes  of, ruffians  calif ornica  X  J.  ref/ia,  Ft:  a,  awn-shaped  trichomes  (  X  84);  b  to  e, 
disk-shaped  trichomes  (6  and  d,  X  800);  c  and  e,  X  535;  /  and  </,  long  secreting  trichomes  (fir,  X 
535);  h,  transverse  section  of  head  of  long  secreting  trichome  of  type  with  more  than  4  cells  in 
head  (  X  535);  i  to  q,  short  secreting  trichomes;  i  ( X  800)  and.?  (X  535),  young  trichomes;  A-,  tri- 
chome from  region  of  vein  of  leaf  (X  535);  I,  trichome  from  dorsal  leaf-surface  (X  535);  m,  longi- 
tudinal section  of  short  secreting  trichome  (X  535);  n  top,  trichomes  from  old  leaf  (X  535);  q, 
transverse  section  of  head  of  short  secreting  trichome  (x  830). 

The  short  secreting  trichome  is  multicellular  and  consists  of  a  2 -celled 
stalk  and  a  head  of  4  cells  placed  radially,  as  in  both  pure  lines.  Beyond 
the  first  division  of  the  young  trichome,  which  is  a  transverse  one,  no  stages 


42 


HEREDITY   AS   ILLUSTRATED   BY   TRICHOMES. 


in  its  development  were  seen.  Measurements  on  the  mature  trichomes 
show  that  the  variation  in  length  of  the  entire  organ,  but  particularly  of  the 
head,  is  in  some  manner  associated  with  the  position  occupied  by  it,  as  has 
been  repeatedly  noted  in  other  forms.  And,  also,  as  in  the  other  similar 
measurements,  this  variation  was  found  to  be  in  a  high  degree  consistent, 
which  is  indicated  by  the  tables  of  measurements  (tables  18  and  19). 

TABLE  18. — Length  of  short  secreting  trichomes  ofjuglans  californica 
X  Juglans  regia,  Fv     (  Trichomes  of  old  leaves.} 


Dorsal 
surface. 

Ventral 
surface. 

Dorsal 
surface. 

Ventral 
surface. 

M 

M 

M 

/* 

46.2 

25.2 

42.0 

29.4 

50.4 

33-6 

37-8 

25.2 

37-8 

25.2 

33-6 

58.8 

25.2 

The  averag-es  in  length  of  trichomes  are:  dorsal  surface,  45.5/*;  ventral, 
28.2  /*.  Similar  results  were  obtained  in  another  series  of  measurements, 
which  need  not  be  given,  in  which  the  dorsally  placed  trichomes  averaged 
35.9  p  and  the  ventrally  placed  ones  28.1  p-  in  length. 

TABLE  19. — Measurements  on  heads  of  short  secreting  trichomes  of  Juglans 
californica  X  Juglans  regia,  Fr     (Trichomes  of  old  leaves}. 


Dorsal 
surface. 

!   Ventral 
surface. 

Dorsal 
surface. 

Ventral 
surface. 

M 
18.9 

21  .O 

16.8 

i 
M 
16.8 
16.8 
16.8 

M 
21  .O 

16.8 
16.8 

M 
12.6 

21.0 

14.7 

The  averages  in  length  of  the  heads  are:  dorsal  surface,  18.9  /*;  ventral 
surface  15.5  ft. 

JUGLANS  HYBRIDS-SECOND  GENERATION. 

The  material  for  the  study  of  ft\&  Juglans  hybrids  and  their  pure  parents, 
which  has  been  reported  on  above,  was  collected  in  the  spring  of  1907. 
This  material,  unless  otherwise  specifically  noted,  was  of  the  first  genera- 
tion. The  study  was  resumed  in  1908  for  the  following-  purposes:  It 
seemed  best  to  obtain  trichomes  which  were  undergoing  nuclear  division 
in  order  to  satisfactorily  determine  the  sequence  of  the  cell-divisions,  and, 
also,  it  seemed  best  to  carry  the  study  of  the  variation  of  the  trichomes 
somewhat  further,  and  to  learn  if  possible  the  relation  between  the  vari- 
ation of  the  trichomes  and  the  variation  of  the  plant  bearing  them. 

In  selecting  the  plants  for  study  it  was  decided  to  take  2  each  of  the 
largest  and  the  smallest  of  both  kinds  of  hybrids.  The  largest  Juglans 


HYBRIDS  OF  JUGLANS.  43 

californica  X  Jug  lam  nigra,  Nos.  1  and  2,  were  both  about  1.5  meters 
high;  the  smallest  of  this  cross,  Nos.  3  and  4,  were  40  and  50  cm.  high, 
respectively.  The  largest  specimens  ol  Juglans  calif  ornica  X  Juglans  regia, 
Nos.  A  and  B,  were  from  3  to  4  meters  high;  and  the  smallest,  Nos.  C  and 
D,  were  70  and  150  cm.  high,  respectively.  All  of  the  second-generation 
plants  of  either  race  were  of  the  same  age  and  were  apparently  growing 
under  similar  conditions. 

LEAVES  OF  THE  HYBRIDS. 

Fully  developed  leaves,  taken  from  analogous  portions  of  the  plants,  were 
examined  and  the  basal  pair  of  leaflets  were  photographed  (plates  9  and 
10).  Plate  9  shows  the  basal  leaflets  of  4  leaves  of  plant  No.  4,  Juglans 
calif  ornica  X  Juglans  nigra.  No.  1  of  the  figure  had  10  leaflets  and  was  18 
cm.  long;  No.  2  had  11  leaflets  and  was  29  cm.  long;  No.  3  had  11  leaflets 
and  was  21  cm.  long;  and  No.  4  had  11  leaflets  and  was  26.5  cm.  in  length. 
Plate  9  shows,  also,  the  basal  leaflets  of  leaves  from  plant  No.  1 .  Of  the  fig- 
ure, No.  1  is  of  a  leaf  which  was  40  cm.  long  and  had  18  leaflets;  No.  2  had 
17  leaflets  and  was  34  cm.  in  length;  No.  3  is  of  a  leaf  which  had  17  leaflets 
also  and  was  36  cm.  long;  and  No.  4  is  of  a  leaf  which  bore  13  leaflets 
and  was  25  cm.  in  length.  Plate  10,  lower  figure,  is  of  the  basal  leaflets  of 
plant  No.  A,  Juglans  calif  ornica  X  Juglans  regia.  Of  the  figure,  No.  1  is  of 
a  leaf  which  bore  11  leaflets  and  was  49.5  cm.  long;  No.  2  is  of  a  leaf  which 
bore  9  leaflets  and  was  51  cm.  long;  No.  3  is  of  a  leaf  which  had  7  leaflets  and 
was  25  cm.  long;  and  No.  4  is  of  a  leaf  which  had  11  leaflets  and  which  was 
55.5  cm.  in  length.  Plate  10,  upper  figure,  is  of  the  basal  leaflets  of  leaves 
from  plant  No.  C.  No.  1  is  from  a  leaf  which  bore  9  leaflets  and  which  was 
34.5  cm.  long;  No.  2  is  of  a  leaf  which  had  7  leaflets  and  was  25  cm.  in 
length;  No.  3  is  of  a  leaf  which  bore  8  leaflets  and  measured  32.5  cm.  in 
length;  and  No.  4  is  of  a  leaf  which  had  8  leaflets  and  which  was  26  cm.  in 
length.  The  leaves  and  leaflets  of  D,  which  are  not  shown,  were  very 
large,  so  that  only  two  leaf -bases  could  be  accommodated  on  the  photo- 
graphic screen  at  one  time.  One  leaf  of  D  had  12  leaflets  and  was  37.5  cm. 
long;  and  another  had  15  leaflets  and  was  66.5  cm.  in  length. 

There  is  thus  a  considerable  variation  in  both  series  in  size  of  leaves 
and  in  number  of  leaflets,  both  as  regards  those  of  a  single  individual 
and  those  from  different  plants  of  the  same  blood.  As  for  other  char- 
acters, such  as  form  of  leaflet,  shape  of  apex  and  of  base,  emargination, 
and  texture  or  character  of  surface,  there  is  less  variation  between  the 
leaves  of  any  individual  than  between  the  leaves  from  different  plants. 
From  the  examination  the  following  generalizations  seem  to  hold:  (l) 
The  largest  leaves  have  also  the  greatest  number  of  leaflets;  (2)  the  largest 
leaves  are  most  variable  as  regards  the  number  of  component  leaflets.  The 
converse  of  these  is  true  for  the  smaller  leaves.  Except  for  plant  D,  also, 
the  largest  leaves  are  borne  upon  the  largest  plants;  that  is,  the  leaves  ap- 
pear to  be  an  index  of  the  vigor  of  the  plants.  The  number  of  leaflets  of 


44  HEREDITY   AS   ILLUSTRATED   BY   TRICHOMES. 

the  leaves  of  both  series  does  not  indicate  distinct  reversion  to  pure  lines 
in  any  instance.  In  Juglans  calif  arnica  X  Juglans  nigra,  in  plant  No.  1, 
the  leaflets  numbered  from  13  to  18;  in  No.  4,  from  10  to  11.  In  the  pure 
line  calif  arnica  the  leaflets  numbered  from  19  to  21,  and  in  nigra  from  13 
to  21.  It  therefore  appears  that  the  hybrid  No.  4,  which  is  small  in  size, 
has  a  smaller  number  of  leaflets  than  were  seen  in  any  leaf  of  either  pure 
line.  \T\Juglans  calif  arnica  X  Juglans  regia,  on  the  other  hand,  in  num- 
ber the  leaflets  are  intermediate,  since  in  A  they  range  from  7  to  11,  and 
in  C  from  7  to  9,  while  in  regia  the  range  is  from  5  to  7.  It  is,  therefore, 
concluded,  as  regards  the  plants  which  were  especially  studied  of  both 
series,  that  they  do  not  show  reversion  in  any  gross  leaf-character  to  pure 
lines  in  any  instance . 

TRICHOMES  OF  THE  HYBRIDS. 

The  material  for  study  was  found  to  be  especially  favorable  for  the  study 
of  the  embryogeny  of  the  trichomes,  and,  accordingly,  an  account  will  be 
given  here  of  the  origin  and  the  development  of  the  leading  types.  It 
should  be  stated  at  the  outset  that  the  cell-divisions  were  seen  in  numbers 
sufficiently  large  to  point  to  the  soundness  of  the  conclusions  based  on 
this  phase  of  the  investigation. 

Five  forms  of  trichomes  were  found  in  the  hybrid  Juglans  calif  arnica  X 
Juglans  nigra  and  4  in  the  other  hybrid.  The  type  not  common  to  both 
is  the  long  secreting  trichome  already  noted  as  occurring  in  Jiiglans  nigra 
and  in  the  first-generation  hybrid  with  nigra  blood.  In  addition  to  these 
trichomes,  which  do  not  require  any  additional  description  here,  3  or  4 
abnormal  types  were  seen,  all  but  one  of  which  had  already  been  noted. 
These  will  be  described  below.  Since  the  development  of  any  type  of  tri- 
chome adheres  to  its  peculiar  pattern  in  whichever  strain  it  is  found,  unless 
especially  stated  to  the  contrary,  the  subjoined  descriptions  apply  to  both 
lines. 

The  disk-shaped  trichome  takes  its  origin  as  a  squat  projection  of  an 
epidermal  cell  and  early  undergoes  transverse  division.  The  first  division 
of  the  unicellular  trichome  thus  formed  is  a  transverse  one  by  which  the 
portion  which  is  to  become  the  head  is  separated  from  the  portion  to 
become  the  supporting  stalk.  The  second  division  is  a  longitudinal  one  in 
the  end-cell.  This  sequence  was  observed  without  exception  in  the  disk- 
shaped  trichome,  and  is  the  sequence  of  the  first  two  cell-divisions  in  all 
of  the  other  multicellular  trichomes,  save  only  a  single  aberrant  type  which 
will  be  mentioned  below.  In  all  cases  examined  the  next  divisions  occur 
in  the  head,  which  appears  to  become  4-celled  at  least  prior  to  the  trans- 
verse cell-division  which  completes  the  divisions  of  the  stalk  ;  and  tri- 
chomes were  observed  with  the  fifth  and  sixth  head-cells  forming,  and  one 
with  ahead  of  8  cells  without  the  final  division  of  the  stalk-cell.  On  the 
other  hand,  trichomes  were  seen  which  had  the  stalk  fully  developed, 


CANNON 


PLATE  9 


L^Bftvlt 


**; 
3 


Basal  leaflets  of  leaves  of  Juglans  californica  X   Juglans  nigra,  F2,  in  plants  "4"  and  "1". 
One-half  natural  size.    Further  explanation  in  the  text 


CANNON 


PLATE  10 


Basal  leaflets  of  leaves  of  Juglans  californica  X  Juglans  regia,  F2,  of  plants  "C"  and  "A". 
One-half  natural  size.     Further  explanation  is  given  in  the  text. 


HYBRIDS    OF   JUGLANS. 


45 


although  the  head  consisted  of  6  cells  only.  It  is  thought  that  there  may 
be  a  relation  between  the  large  number  of  cells  of  which  the  mature  tri- 
chome  is  composed  and  the  lack  of  consistency  in  the  sequence  of  its  cell- 
divisions.  The  head  of  the  mature  disk-shaped  trichome  is  composed  of 
at  least  32  cells. 


PIG.  11. — Trlchomes  ofJuylans  californica  X  J>  nigra,  F2,  plant  No.  1:  a,  figures 
showing  the  cutting  off  of  the  young  trichome  from  the  epidermis  of  the 
leaf;  b,  first  cell-division  of  trichome;  c,  second  or  longitudinal  division  of  tri- 
chome. (All  figures  X  1200.) 

As  opposed  to  the  want  of  uniformity  in  the  cell-division  sequence  shown 
in  the  disk-shaped  trichome,  the  divisions  in  the  short  and  the  long 
secreting  trichomes  follow  a  perfectly  consistent  sequence  throughout. 
After  the  short  secreting  trichome  is  cut  off  from  the  epidermis  it  under- 
goes division  by  a  transverse  wall  into  a  terminal  cell,  which  will  give  rise 
to  the  head,  and  a  basal  portion,  the  stalk.  The  second  division  occurs  in 
the  end-cell  and  is  a  longitudinal  one.  The  third  division  of  the  trichome 
is  a  transverse  one  in  the  stalk,  by  which  the  divisions  in  the  stalk  region 
are  completed.  The  fourth  and  the  fifth  cell-divisions  are  longitudinal 


46 


HEREDITY   AS   ILLUSTRATED   BY   TRICHOMES. 


ones  in  the  head-cells.  The  trichome  then  consists  of  a  head  of  4  cells, 
radially  disposed,  and  a  stalk  of  2  cells,  and  cell-division  ceases.  Although 
numerous  examples  of  each  stage  in  the  development  of  the  trichome  were 
seen,  no  exception  to  the  sequence  as  given  was  noted. 

The  long  secreting  trichome  in  Juglans  calif  arnica  X  Juglans  nigra  is  of 
two  sorts.     One  type  has  a  stalk  of  4  cells  and  a  head  of  4  cells,  and  the 


FIG.  12.— Long  and  short  secreting  trichomes  of  Juglaius  californica  X  Juglans  nigra,  F2,  plant  1: 
a,  third  cell-division  of  trichome;  b,  4-celled  trichome,  of  which  only  3  cells  are  shown;  c,  fourth 
cell-division  of  trichome;  d,  fifth  cell-division  shown  in  transverse  and  longitudinal  sec- 
tions. (All  figures  X  1200.) 

other  type  has  a  stalk  of  about  8  cells  and  a  head  of  about  8  cells.  The 
development  of  the  first  kind  is  as  follows:  The  first  cell-division  of  the 
yoimg  trichome  is  transverse,  by  which  the  portion  which  is  to  become 
the  head  is  differentiated  from  the  portion  which  is  to  become  the  support- 
ing stalk.  The  second  division  is  longitudinal  in  the  head,  the  third  is 


HYBRIDS  OF  JUGLANS. 


47 


transverse  in  the  stalk,  the  fourth  and  fifth  are  longitudinal  in  the  head. 
To  this  point  the  sequence  of  divisions  of  the  short  secreting-  and  long 
secreting  trichomes  is  the  same.  The  sixth  division  occurs  in  the  upper 
stalk-cell  and  is  transverse,  and  the  seventh  and  last  cell- division  is  a  cross- 
wall  in  the  lower  stalk-cell.  The  successive  divisions  are  illustrated  by  the 
accompanying  figures,  which  also  indicate  the  sequence  of  cell-divisions 
in  the  short  secreting  trichome. 

The  results  of  the  study  on  the  second  type  of  long  secreting  trichome, 
while  not  entirely  satisfactory,  indicate  that  it  is  a  fundamentally  different 
form  than  the  type  with  a  smaller  number  of  cells.  The  youngest  stages 
of  this  trichome  either  were  not  seen  or  could  not  be  identified  as  belong- 


f 


FIG.  13. — Trichomes  of  Juglans  californlca  X  Juglans  nigra,  F2,  plant  1:  a  to  c,  long 
secreting  trichomes,  showing  sequence  of  cell-division  of  stalk;  d  to  /,  some 
stages  in  development  of  short  secreting  trichomes.  ( a  to  c,  X  1200 ;  c,  X  840 ;  d  to 
/,  X  1200.  Reduced  one-fourth.) 

ing  surely  to  the  trichome.  A  trichome  with  5  cells,  of  which  2  were  of 
the  head,  had  undergone  division  of  the  lower  of  the  2  stalk-cells.  An- 
other trichome  having  a  head  of  4  cells  had  just  experienced  the  division 
of  the  lower  stalk-cell.  That  there  is  some  irregularity  in  the  sequence  of 
division  is  shown  by  another  trichome  in  which  the  stalk  consists  of  4  cells, 
although  the  third  cell-division  of  the  head  is  just  taking  place.  The  cell- 


48 


HEREDITY   AS   ILLUSTRATED   BY   TRICHOMES. 


divisions  of  the  head  and  of  the  stalk  appear  to  take  place  independently, 
which  is  especially  and  perhaps  solely  true  of  rather  old  trichomes.  For 
example,  a  trichome  was  seen  which  had  a  head  with  7  nuclei,  but  a  stalk 
with  the  fifth  cell  just  being:  cut  off,  and  another  trichome  was  seen  which 
was  forming'  the  fifth  or  sixth  cell  in  the  head,  although  the  stalk  con- 
sisted of  9  cells. 


FIG.  14. — Disk-shaped  trichomes  of  Juglans  cali/ornica  X  Juglans  nigra,  F2,  plant  1:  a,  trichome 
with  head  consisting  of  4  cells,  of  which  2  are  in  the  process  of  dividing,  and  a  1-celled  stalk;  6, 
trichomes  with  head  of  6  or  7  cells,  and  a,  stalk  of  1  cell  which  is  undergoing  division;  c,  tri- 
chome with  1-celled  stalk  and  4-celled  head;  d,  trichome  with  2-celled  stalk;  e,  transverse  sec- 
tion of  head  to  show  the  formation  of  octants;  /,  trichome  with  a  head  of  7  or  8  cells  and  a 
1-celled  stalk.  (All  figures  X  1200.) 

Additional  evidence  as  to  irregularity  in  the  number  of  cells  making:  up 
the  larger  type  of  long  secreting  trichome  was  observed  in  the  unusually 
large  number  of  head-cells.  A  trichome  was  seen,  and  is  shown  in  fig.  16, 
which  had  a  head  of  14  cells,  although  there  were  only  6  cells  in  the  stalk. 


HYBRIDS  OF  JUG  LANS. 


49 


The  reasons  for  this  curious  behavior  are  not  apparent,  but  an  explanation 
may  possibly  be  found  in  the  following-  facts:  The  trichome  in  question  was 
seen  only  on  the  veins  and  is  the  last  of  the  types  to  appear,  that  is,  the 
older  leaves  only  showed  its  presence,  although  exactly  when  in  the  de- 
velopment of  the  leaf  this  type  was  formed  was  not  learned.  Again,  the 


FIG.  15.— Trichomes  of  Juglans  californica  X  Juglans  nigra,  F  2 ,  plant  4.  ( Unless  otherwise 
stated  the  trlchomes  are  of  the  type  which  has  a  head  of  more  than  4  cells.)  a,  third  cell- 
division  of  trichome;  b,  long  secreting  trichome  undergoing  thp  sixth  cell-division;  c, 
second  division  of  the  stalk,  which  thus  occurs  in  different  sequence  than  in  the  regular 
type;  d,  same  stage  as  before;  e,  2  trlchomes,  showing  stalk  of  4  cells  and  the  third  division 
of  the  head ;  /,  trichome  with  a  head  of  8  cells,  of  which  3  were  seen  in  the  adjoining  sec- 
tion, and  the  fifth  stalk-cell  in  the  process  of  being  cut  oft':  g,  abnormal  trichome  of  long 
secreting  type.  (All  figures  X  1200.) 

great  irregularity  may  be  owing  to  the  hybrid  nature  of  the  plant,  as  no 
such  inconsistencies  were  observed  in  the  pure  strain  nigra.  Or,  the  fact 
that  the  number  of  cells  composing  the  trichome  is  relatively  large  may  be 
a  factor  which  should  be  considered  in  this  connection.  This  irregularity 
is  all  the  more  striking  because  of  the  perfect  consistence  in  the  sequence 
of  cell-divisions  which  was  observed  in  the  smaller  type  of  long  secreting 
trichome  and  in  the  short  secreting  trichome  as  well, 


50 


HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 


Aberrant  and  abnormal  types  of  trichomes  were  seen  in  both  lines  of 
hybrids.  These  either  were  modifications  of  existing1  trichome  forms  or 
quite  new  types.  Disk-shaped  trichomes  with  more  than  2  cells  in  the 
stalk,  and  short  secreting-  trichomes  with  stalks  with  3  in  place  of  2  cells 
were  observed.  The  first  abnormality  noted  was  seen  also  in  the  pure 
parents,  nigra  and  regia.  The  abnormal  short  secreting-  trichome  has  been 
noticed  only  in  the  hybrid  Juglans  calif ornica  X  Juglans  nigra  plants  num- 
bered 1  and  2.  Abnormal  long  secreting-  trichomes  were  seen,  as  above 
indicated,  but  none  which  were  certainly  of  the  smaller  form  of  this  trichome. 


FIG.  16.— Trichome  of  Juglans  calif ornicaY.  Juglans  nlr/ra,  F2,  plant  4. 
Abnormal  form  of  type  of  trichome  which  normally  has  a  head  of  8 
and  a  stalk  of  8  cells.  ( X  1200). 

An  additional  type  of  the  abnormal  trichome,  the 
irregularity  of  which  is  not  dependent  on  the  modifica- 
tion of  a  form  now  prevalent,  was  observed  several 
times  in  Juglans  californica  X  Juglans  nigra  and  once 
in  the  other  hybrid.  This  trichome  consists  of  a  cell- 
complex  in  which  the  customary  supporting  stalk  is 
wanting;  the  trichome  is  made  up  of  3  tiers  of  cells,  of 
which  each  tier  has  2  (fig.  18).  Thus  there  are  6  cells 
in  the  trichome,  of  which  2  are  terminal,  2  basal,  and  2  median.  This 
trichome  arises  as  follows:  After  the  young  organ  is  separated  by  a  cross- 
wall  from  the  epidermis  it  undergoes  longitudinal  division  (fig.  18,  a),  in 
place  of  a  transverse  wall,  as  commonly  is  the  case.  The  circumstance  that 
the  first  wall  is  longitudinal  necessarily  affects  all  subsequent  cell-divisions 
in  such  a  manner  as  to  bring  about  the  origin  of  a  new  form  of  trichome. 
It  is  of  interest  to  note  that  the  trichome  is  not  a  modification  of  an  exist- 
ing type,  but  that  it  appears  suddenly  and  owes  its  existence  to  the  unique 
placing  of  the  first  wall.  It  is  of  interest  to  note  further  that  should  experi- 
mentation show  that  each  type  of  trichome  is  a  unit  character,  we  have 
here  the  origin  of  a  character  through  mutation,  in  place  of  through  gradual 
change,  as  may  be  the  case  with  certain  of  the  other  trichomes  whose 
origin  and  development  have  been  reported  on  in  this  paper. 


HYBRIDS   OF   JUGLANS. 


51 


FIG.  17. — Short  secreting trichomes  of  Juglans californica  X  Juglants  reffia,F2,  plant C:  a  toe,  origin 
and  growth  of  trichome  previous  to  separation  from  epidermis  (X  840)';  d and  e,  separation  of 
trichome  rudiment  from  epidermis  (X  1200j;/,  1-celled  stage  (X  840);  y,  first  divison  of  trichome 
(X 1200);  /*.,  2-celled  trichome  ( X  840);  i,  second  division  of  trichome;  j,  3-celled  trichome  ( X  1200); 
k,  third  cell-division  (x  1200);  I,  transverse  section  of  leaf,  showing  long  and  short  secreting  tri. 
chomes  in  place, 


52 


HEREDITY   AS   ILLUSTRATED   BY   TRICHOMES. 


MEASUREMENTS  ON  THE  TRICHOMES. 

The  measurements  which  were  made  on  the  multicellular  trichomes  of 
the  pure  lines  and  first  generation  of  Juglans  hybrids  indicate,  as  above 
shown,  considerable  variation  in  size,  but  comparative  study  showed  also 
that  the  causal  relations  attending"  the  variations  might  be  traced,  or  at 
least  surmised.  Thus  the  variations  were  seen  to  be  associated  with  the 
age  of  leaf  or  with  the  position  occupied  by  the  trichomes  on  the  leaf. 
These  conditions  were  shown  to  hold  good  for  all  the  plants  brought  under 

observation.  It  followed, 
therefore,  when  these  facts 
were  taken  into  account,  that 
comparison  between  closely 
related  forms  was  possible. 
As  the  studies  reported  on 
up  to  page  42  were  made  in 
two  successive  seasons,  1906 
and  1907,  there  remained  the 
possibility  that  variations  in- 
duced by  seasonal  differences 
might  in  some  degree  alter 
the  measurements  as  given 
in  the  earlier  studies  by  modi- 
fying the  development  of  the 
hybrids  whose  ancestors  va- 
ried in  climatic  experience  as 
well  as  in  blood.  There  also 
remained  an  instance  of  the 
behavior  of  trichomes  in  re- 
version to  the  pure  lines  to 
be  observed,  since  no  hybrid 
examined  during  the  earlier 


FIG.  18. — Abnormal  and  aberrant  forms  of  trichomes  of 
Juglans  californica  X  Juglans  nigra,  F2,  plant  1:  a, 
young,  and  b  and  c  mature  stages  of  a  new  type  of 
trichome  (the  first  cell-division  of  the  trichome  is  lon- 
gitudinal in  place  of  being  transverse,  as  is  the  usual 


position);  d  and  e,  abnormal  trichomes.  (a,  b,  and  c, 
X1200;  d,  X  840;  e,  X  535.) 


course  of  this  study  exhibited 
this  phenomenon.  The  meas- 
urements to  be  given  presently,  therefore,  are  calculated  especially  to 
show  two  thing's,  namely,  (l)  whether  the  trichomes  exhibit  seasonal 
variation  in  size,  and  (2)  whether  ancestral  characteristics  relating  to 
size  of  trichomes  reappear  in  the  second  hybrid  generation. 

JUGLANS  CALIFORNICA  X  JUGLANS  NIGRA,  SECOND  GENERATION. 

Of  the  several  forms  of  multicellular  trichomes,  measurements  were  made 
on  the  disk-shaped  and  on  short  secreting  types  only,  and  of  these  the 
latter  offered  most  favorable  material  and  was  mostly  used. 

The  following  measurements,  in  /*,  were  derived  from  disk-shaped  tri- 
chomes taken  from  the  largest  and  from  the  smallest  of  the  hybrids.  They 


HYBRIDS  OF  JUG  LANS. 


53 


are  of  diameters  only;  it  was  found  not  practicable  to  make  a  special 
study  of  the  depth  of  the  head.  Disk-shaped  trichomes  from  No.  4, 
diameters  of  heads:  79.8,  84,  84,  79.8,  92.4,  100.8,  71.4;  the  average 
diameter  is  84.6.  The  following-  are  the  diameters  of  disk-shaped  tri- 
chomes from  No.  1:  84,  79.8,  79.8,  84,  84,  79.8,  79.8,  79.8,  71.4,  84,  79.8, 
71.4;  the  average  is  79.8.  The  disk-shaped  trichomes  from  the  smaller 
plant,  No.  4,  average  larger  than  those  from  the  larger  one,  No.  1.  In 
both  instances  the  trichomes  run  smaller  than  in  the  first  generation 
( 101.1  A*),  and  also  smaller  than  in  the  pure  species  Juglans  calif  arnica 
(107  /*),  and  appear  to  be  comparable  to  the  trichomes  of  the  pure  species 
Juglans  nigra;  that  is,  there  appears  in  this  case  to  be  a  nigra  reversion. 
The  short  secreting  trichomes  were  most  numerous,  so  that  accordingly 
the  measurements  on  them  were  most  complete.  Both  of  the  hybrids 
from  which  the  disk-shaped  trichomes  just  reported  on  were  taken  were 
examined  as  regards  the  short  secreting  trichomes.  On  the  trichomes  the 
following  series  of  measurements  were  taken:  (l)  length  of  entire  organ, 
(2)  length,  and  (3)  diameter  of  the  head.  The  relative  age  of  the  leaf 
and  the  position  occupied  on  it  are  both  recorded  in  table  20. 

TABLE  20. — Measurements  on  short  secreting  trichomes,  hybrid  No.  /.* 


Dorsal  surface. 

Ventral  surface. 

Entire 

Length 

Diameter 

Entire 

Length 

Diameter 

length. 

of  head. 

of  head. 

length. 

of  head. 

of  head. 

M 

v- 

M 

M 

^ 

M 

36.6 

18.8 

29.28 

20.13 

12.  8l 

20.1} 

4i.24(v) 

21  .96 

27.45 

3i.n(v) 

14.64 

2O.  13 

28.62 

16.47 

20.13 

27-45 

16.47 

21  .96 

30.  10 

16.47 

18.30 

27-45 

16.47 

23.79 

34-77 

18.30 

25.62 

21  .96 

12.  8l 

18.30 

32-94 

18.30 

23-79 

32-94(v) 

21  .96 

21  .96 

31  .  ii 

2O.  13 

21.96 

29.28  (v) 

18.30 

23-79 

32-94 

20.13 

21  .96 

23-79 

12.  8l 

18.30 

29.28 

16.47 

21  .04 

29.28(V) 

14.64 

21  .96 

36.6  (v) 

25.62 

29.28 

31.11  (v) 

16.47 

25.62 

29.28  (v) 

14.64 

21.96 

23-79(v) 

10.98 

21.96 

23-79(v) 

14.64 

21  .96 

*In  this  and  subsequent  tables  (v)  refers  to  the  location  on  a  vein;  where  the 
letter  is  not  given  the  trichome  is  understood  to  be  placed  between  the  veins. 

The  averages  of  these  measurements  are  as  follows:  Dorsal  surface,  be- 
tween veins,  entire  length,  32  /*;  length  of  head,  18  ^;  diameter  of  head, 
24.7  p.  Ventral  surface,  between  veins,  entire  length,  24  /*;  length  of 
head,  14.2;  diameter  of  head,  20.5  M;  on  veins,  entire  length,  28.7  A4;  length 
of  head,  15.9  p;  diameter  of  head,  22.5  /*. 

The  leaves  of  both  series  of  hybrids  were  not  entirely  mature;  they  were 
in  each  instance  84  /*  in  thickness. 


54 


HEREDITY    AS    ILLUSTRATED    BY    TRICHOMES. 


Another  study  on  the  dimensions  of  the  trichomes  of  No.  4,  as  will  ap- 
pear directly,  gave  somewhat  different  results.  The  leaves  were  of  the 
same  thickness  as  those  reported  on  above.  The  measurements,  of  which 
there  were  60,  may  be  disregarded;  the  results  are  as  follows:  Dorsal  sur- 
face, entire  length,  31.47  P,  length  of  head  16.5  /*,  diameter  of  head  21.6  /*•; 
ventral  surface,  entire  length,  29.2  /*;  length  of  head  16.2  /*,  diameter  of 
head  22.6  p.  Thus  the  length  of  the  trichomes  from  the  dorsal  surface  is 
about  the  same  in  both  cases,  but  the  size  of  the  heads  is  in  the  latter 
instance  somewhat  greater.  The  dimensions  of  trichomes  from  the  ventral 
surface  of  the  former  are  nearly  the  same  as  those  from  between  the  veins 
of  those  last  given.  In  each  instance  the  same  relation  of  length  to 
diameter  of  head  is  almost  exactly  held. 

TABLE  21. — Measurements  on  short  secreting  trichomes,  hybrid  No.  4.. 


Dorsal  surface. 

Ventral  surface. 

Entire 

Length 

Diameter 

Entire 

Length 

Diameter 

length. 

of  head. 

of  head. 

length. 

of  head. 

of  head. 

M 

u 

M 

M 

ft.                       n 

36.6  (v) 

18.3 

23-79 

27-45(v) 

10.98 

14.64 

36.6  (v) 

18.3 

23.79       ;           29.28 

12.81           10.98 

*4i.26 

14.64 

23-79                 36'6 

18.3            21.96 

36.6 

i3-3 

27-45 

3i.u(v) 

I  2  .  8  I                  2  I  .  96 

36.6 

14.64 

25.62 

*32.9l 

16.47                 20.13 

38-4  (v) 

18.30 

20.  13 

34-77(v) 

23.79                 20.13 

38.43                16.47 

21.96 

42.09(7) 

14.64 

18.30 

t43-92(v)          20.13 

25.62 

t3i.ii 

12.  8l 

18.30 

*53-7 

20.  13 

23-79 

$49.41 

14.64 

21  .96 

27-45 

14.64 

23-79 

31.11 

14.64 

21  .96 

31.11 

14.64 

20.13 

•Attached  to  leaf-surface  in  close  relation  to  a  vein. 

tThese  trichomes  occurred  on  the  same  vein. 

{These  trichomes  were  on  opposite  sides  of  the  leaf  and  very  near  a  vein. 

The  averages  are  as  follows:  Dorsal  surface,  entire  length,  between  veins, 
33.8  A*,  length  of  head  16.44  p-,  diameter  of  head  25  )«•;  on  veins,  entire  length 
41.84  /*,  length  of  head  18.3  /*,  diameter  of  head  23.5  A*;  ventral  surface 
between  veins,  entire  length  31.1  p-,  length  of  head  19  /"-,  diameter  of  head 
19.70  /*•;  on  veins,  entire  length  35.5  /«•,  length  of  head  15.16  /*,  diameter 
of  head  19.26  /*. 

The  leaves  studied  were  not  fully  developed;  they  were  84  A*  in  thickness. 
If  we  compare  the  average  measurements  of  hybrids  1  and  4  we  shall  get 
the  following  results:  (l)  trichomes  taken  on  the  dorsal  surface,  between 
veins,  are  longer  in  plant  4  than  in  plant  1,  and  the  heads  also  are  larger, 
but  they  retain  similar  proportions;  (2)  those  between  the  veins  on  the 
ventral  surface  of  plant  4  are  also  longer  than  in  1,  and  the  heads  are 
longer  but  of  less  diameter  (californica  character  ?);  (3)  on  the  veins  the 
trichomes  of  4  are  larger  than  those  on  the  veins  of  1 ,  but  the  heads  are 
shorter  and  of  less  diameter,  that  is  to  say,  the  trichomes  of  hybrid  No.  4 


HYBRIDS  OF  JUGLANS. 


55 


are  uniformly  larger  than  those  of  hybrid  No.  1,  which  is  a  larger  plant. 
The  form  of  the  heads  is,  with  one  exception,  the  same  in  both  series  of 
plants . 

It  has  been  noted,  in  the  tables  and  the  averages,  that  the  heads  of  the 
trichomes  are  wider  than  long,  and  this  also  is  a  character  that  distinguishes 
nigra  from  calif ornica .  The  relative  form  of  the  heads  of  the  short  secret- 
ing trichomes  in  the  pure  species  and  the  form  of  the  head  in  this,  the 
second-generation  plant  No.  4,  are  shown  in  the  figures.  Therefore  it 
appears  that  we  here  meet  a  well-defined  instance  of  reversion  to  the  pure 
species  nigra. 

JUGLANS  CALIFORNICA  X  JUGLANS  REGIA,  SECOND  GENERATION. 

Measurements  were  made  on  trichomes  taken  from  the  leaves  of  plants 
A  and  C,  which  were  among  the  largest  and  smallest,  respectively,  of  those 
of  this  generation.  The  study  was  carried  out  mainly  on  the  short  secret- 
ing trichomes,  for  the  reason  that  this  type  was  most  numerous,  although 
some  measurements  also  were  made  on  the  disk-shaped  trichomes.  The 
long  secreting  trichomes  in  the  material  examined  were  not  present  in 
numbers  large  enough  to  make  a  satisfactory  study  of  them. 

TABLE  22. — Measurements  on  short  secreting  trichomes,  hybrid  No.  A . 


Dorsal  surface. 

Ventral  surface. 

Entire 

Length 

Diameter 

Entire           Length 

Diameter 

length. 

of  head. 

of  head.          length.         of  head. 

of  head. 

/"• 

u, 

M 

yu,                        /* 

/* 

36.6  (v) 

18.3 

36.6 

25.62  (v)          12.  81 

29.28 

62.22  (v) 

34-77 

36.6 

25.62  (v)          16.47 

25.62 

32-94  (v) 

18.3 

29.28 

25.62                  12.  81 

20.  13 

32-94 

,8.3 

32.94           31.11   (v)          14.64 

27-45 

36.6  (v) 

18.3 

27.45           25.62                  12.  81 

27-45 

29.28 

20.13 

29.28           25.62                  12.  81 

29.28 

34-77(v) 

16.47 

21.96          31.11  (v)          16.47 

27-45 

32-94 

16.47 

32.94           29.28  (v)          14.64 

29.28 

32.94 

18.3 

31.11           27.45                  12.  81 

27-45 

34-77 

20.13 

32.94        34.77            18.30 

32.84 

40.26  (v) 

25.62 

40.26 

25.62                  12.  81 

27.45 

36.6 

18.3 

32-94 

29.28                16.41 

25.62 

34-77 

18.3 

31.11                 21.  96                          12.  8  1 

27.45 

34-77 

20.  13 

31-4 

27.45              14.64 

27-45 

27-45 

16.47 

21  .96 

29.28  (v)          16.47 

27-45 

38.45(v) 

20.13 

32-94 

25.62                16.47 

25.62 

The  averages  are  as  follows:  Dorsal  surface,  entire  length,  between 
veins  32.94  p,  length  of  head  18.5  /*,  diameter  of  head  30.4  /*;  on  veins, 
entire  length  40.2  /*,  length  of  head  21.7  /*,  diameter  of  head  32.1  /*; 
ventral  surface,  between  veins,  entire  length  27.04  p-,  length  of  head 
14.4 /*,  diameter  of  head  27.8  /*;  on  veins,  entire  length  28.67  /*•,  length 
of  head  15.25  p,  diameter  of  head  27.75  /*. 


56 


HEREDITY  AS  ILLUSTRATED  BY  TRICHOMES. 


Following-  are  the  diameters,  in  p,  of  disk-shaped  trichomes  from  leaves 
of  plant  A:  54.6,  67.2,  67.2,  63,  58.8,  67.2,  58.8,  67.2,  71.3,  63,  71.3,  67.2, 
71.3,  71.3,  71.3,  which  average  66  p.  The  following  dimensions,  in  p-, 
were  obtained  of  disk-shaped  trichomes  from  C:  67.2,  71.3,  67.2,  67.2, 
67.2,  63,  63,  67.2,  71.3,  71.3,  67.2,  which  average  67.5  /*.  The  diameters 
of  the  trichomes  of  the  larger  plant  A  and  those  of  the  smaller  plant  C 
are  nearly  the  same.  The  trichome  appears  not  to  revert  to  either  pure 
line,  but  holds  an  intermediate  position,  as  in  the  first  generation. 

Dimensions  of  the  short  secreting  trichome  of  both  of  the  plants  A  and 
C  were  especially  noted.  These  included  the  entire  length  of  the  trichome, 
the  length,  and  the  diameter  of  the  head.  Attempt  was  made  to  get 
leaves  which  were  of  approximately  the  same  age,  but  it  happened  that 
those  of  A  were  10  p  thicker  than  those  of  C.  The  former  was  94  p  and 
the  latter  84  /*  in  thickness.  It  will  be  recalled  that  the  leaves  of  hybrids 
numbered  1  and  4  were  also  84  p  in  thickness. 

The  measurements  show  that  the  length  of  the  trichomes  of  the  dorsal 
surface,  as  well  as  both  dimensions  of  the  heads  of  trichomes  from  this  sur- 
face, are  larger  than  the  corresponding  dimensions  of  trichomes  from  the 
opposite  leaf -surf  ace,  and  also  that  all  of  the  dimensions  of  trichomes 
from  the  veins  are  greater  than  the  dimensions  of  trichomes  from  between 
them.  The  fact  of  reversion  is  not  clear.  The  diameter  of  the  heads  of 
the  hybrid  trichomes  is  considerably  greater  than  that  of  either  californica 
or  regia,  and  the  length  of  the  head  is  intermediate  between  the  lengths 
of  the  heads  of  this  trichome  in  the  two  pure  species.  The  proportions  of 
the  head  of  the  hybrid,  on  the  other  hand,  are  practically  the  same  as  the 
proportion  in  the  pure  species  regia.  It  is  to  be  noted  that  the  form  of  the 
head  is  nearly  as  in  the  first  generation  of  this  hybrid  (fig.  19). 

TABLE  23. — Measurements  on  short  secreting  trichomes,  hybrid  No.  C. 


Dorsixl  surface. 

Ventral  surface. 

Entire 

Length 

Diameter 

Entire 

Length 

Diameter 

length. 

of  head. 

of  head. 

length. 

of  head. 

of  head. 

M 

M 

M 

M 

/* 

M 

36.6  (v) 

18-3 

27.45               25.62 

16.47 

25.62 

31.11 

16.47 

27.45            29.28(v) 

14.64 

29.28 

34-77 

18.3 

23.79         27-45 

14.64 

27.45 

45-75(v) 

20.13 

29.28           36.6  (v) 

21  .96 

27-45 

4o.26(v) 

18.3 

23-79            43-92  (v) 

21  .96 

34-77 

42.09(v) 

20.13 

27-45 

32-94 

20.13 

23-79 

36.6  (v) 

30-13 

29.28 

27.45                   16.47 

31.11 

4O.26(v) 

21  .96 

23-79 

27-45(v)         04.64 

21  .96 

27-45(v) 

I8.3 

2  I  .  96 

34-77(v)          18.3 

27-45 

36.6 

20.13 

29.45 

34-77(v)          18.3 

29.28 

36.6    (v) 

16.47 

25.62 

23.79                10.96 

23-79 

3»-43(v) 

20.13 

32-94 

21.96                14.64 

21  .96 

34-77 

I8.3 

29.28 

31.11                 i  4  .  64 

25.62 

34-77  (v) 

I8.3 

31.11 

31.11                 16.47 

29.28 

29.28 

14.64 

25.62 

42.09(v)          23.79 

27-45 

32-94(v) 

16.47 

i 

20.  14 

32.94(v)         18.3 

29.29 

HYBRIDS  OF  JUGLANS. 


57 


The  averages  of  the  measurements  are  as  follows:  Dorsal  surface  be- 
tween veins,  entire  length  33.5  p-,  length  of  head  17.6  p-,  diameter  of  head 
27.4  p-;  on  veins,  entire  length  37.69  p-,  length  of  head  19  p-,  diameter  of 
head  26  p-;  ventral  surface,  between  veins,  entire  length  27. 6 p-,  length  of 
head  15.5  P-,  diameter  of  head  26  p-;  on  veins,  entire  length  35.2  p-,  length 
of  head  18.9  /»,  diameter  of  head  28.3  p-. 

From  the  averages  it  appears  that  the  trichomes  which  occur  on  the 
veins  are  larger  than  those  between  them,  and  that  those  on  the  dorsal 

surface  are  larger  than  those 
on  the  ventral .  There  is  very 
little  difference  between  the 
measurements  of  trichomes  of 
plant  A,  which  is  among  the 
largest  of  those  of  the  second 
generation,  and  of  plant  C, 
which  is  of  the  smallest  of  this 
generation.  The  difference  is 
so  slight  that  the  sum  of  the 
averages  of  all  dimensions  of 
the  trichomes  of  A  are  only  5 
more  than  the  sum  for  C .  The 
form  of  the  head  is  the  same 
as  in  A. 

In  hybrid  numbered  A,  so  in  C,  the  fact  of  reversion  to  the  pure  lines 
is  not  clear.  The  heads  of  the  trichomes  of  the  hybrid  are  broader  as  well 
as  shorter  than  those  of  regia,  but  they  are  smaller  in  these  dimensions 
than  the  heads  of  calif ornica.  This  condition  is  shown  diagrammatically 
by  fig.  19,  which  is  based  on  the  average  measurements  of  the  two  pure 
lines  and  the  hybrid.  It  will  be  seen  that,  in  form  at  least,  the  heads  of 
the  trichomes  of  the  hybrid  agree  very  well  with  the  pure  species  regia,  to 
which  they  appear  to  revert. 

GENERAL  COMPARISON  OF  TRICHOMES   IN  THE  PURE  SPECIES 
AND  HYBRIDS  OF  JUGLANS. 

The  study  of  the  trichomes  of  the  Juglans  hybrids  and  pure  species  has 
been  carried  on  with  the  purpose  of  comparing  not  only  the  mature  organs 
of  the  hybrids  with  those  of  the  parents,  but  also  their  origin  and  devel- 
opment step  by  step  so  far  as  possible.  As  the  study  progressed  it  was 
found  that  material  for  comparison  of  mature  structures  was  abundant 
enough,  but  material  for  the  observation  of  the  developmental  stages, 
which  depends  on  the  presence  of  mitotic  figures,  was  not  at  all  easy  to 
get,  so  that,  as  the  second-generation  hybrids  were  fairly  rich  in  this 
particular,  the  account  of  the  development  of  the  trichomes  is  practically 
confined  to  plants  of  this,  the  second,  generation. 


FIG.  19.— Diagrams,  based  on  numerous  measurements, 
showing  reversion  of  the  heads  of  short  secreting  tri- 
chomes in  the  second-generation  Juglans  hybrids, 
a,  .7".  californica  X  J.  regia,  plant  A.  Legend:  solid 

line  ( ),  J.  califcrnica:  dotted  line  ( ),  J. 

re(/ia;  broken  line  ( ),  hybrid.   b,  J.  califor- 
nica X  J.  nif/ra,  plant  1,    Legend:  solid  line  ( ) 

J.  californica;  dotted  line  ( ),  J.  nigra:  broken 

line  ( ),  hybrid. 


58 


HEREDITY    AS    ILLUSTRATED    BY    TRICHOMES. 


4-15' 


Four  types  of  trichomes  were  seen  in  the  pure  species  californica,  nigra, 
and  regia,  and  in  the  hybrids  Juglans  californica  X  Juglans  nigra  and 
Jug  lam  californica  X  Juglans  regia,  but  in  the  species  nigra  and  the  hybrid 
into  which  it  enters  an  additional  form  of  trichome,  not  observed  in  the 
other  plants,  was  also  seen.  In  addition  to  these,  the  regular  types,  there 
were  also  abnormal  forms  of  trichomes  which  were  clearly  modifications 
of  those  more  commonly  present,  and  one  type  which  was  essentially 
different  from  those  most  abundant.  With  one  exception  all  types  of 
trichomes  were  multicellular  and  glandular. 

In  the  second  generation  of  the  hybrid  Juglans  californica  X  Juglans  nigra, 
two  of  the  multicellular  trichomes,  namely,  the  short  and  the  long"  secret- 
ing- trichomes,  are  composed  of  6  and  8  cells, 
respectively.  The  sequence  of  cell-divisions 
up  to  the  formation  of  6  cells  is  identical, 
hence  the  larger  trichome  may  be  regarded 
as  a  further  developed  short  secreting  one, 
or  vice  versa,  the  smaller  type  may  be  held  to 
represent  an  arrested  stage  in  the  develop- 
ment of  the  long  secreting  trichome  (fig.  20). 
It  does  not  seem  probable  that  both  have 

FIG  20        sequence  in  cell-division    descended    from    a    common    ancestor  (so    to 
in  the  short  and  the  long  secreting 

trichomes  of  jugians.  The  num-   speak)  which   had  intermediate  characters, 

bers  give  the  succession  of   cell-       {  tfa      simplicity    of   the    structure    Seems 

will  formation  in  proper  sequence. 

to  preclude  this.  Of  the  two  other  types  of 

multicellular  trichomes  which  occur  in  the  hybrid,  and  which  have  from 
16  to  32  or  more  cells,  the  disk-shaped  trichome  agrees  in  the  sequence  of 
its  divisions  with  the  two  trichomes  mentioned  above  in  the  first  two 
divisions  only,  and  the  fourth  type  of  multicellular  trichome  agrees  in 
sequence  with  the  two  trichomes  mentioned  in  the  first  3  and  possibly  the 
first  5  cell-divisions. 

The  sequence  of  all  of  the  cell-divisions  in  the  6-celled  and  the  8-celled 
trichomes  was  found,  by  repeated  observations,  to  be  perfectly  consistent, 
and  the  first  two  or  first  three  divisions  also  in  the  larger  trichomes  were 
seen  to  be  consistent  for  the  particular  type  of  trichome,  but  the  subsequent 
cell-divisions  of  the  larger  trichomes  appeared  not  to  occur  in  consistent 
sequence.  The  last  statement  is  made  with  reservation,  however,  as 
further  and  careful  study  of  later  stages,  which  would  be  a  difficult  task, 
might  reveal  an  unexpected  regularity  of  cell-division  in  such  trichomes. 

The  abnormal  trichomes  were  of  the  following  types:  they  were  short 
secreting  trichomes  with  one  additional  stalk-cell,  or  a  disk-shaped  tri- 
chome with  1  or  2  additional  cells  in  the  stalk.  There  seemed  also  to  be 
abnormal  forms  of  the  long  secreting  trichome  in  which  the  stalk  had  an 
unusually  large  number  of  cells  and  the  head  also  had  a  larger  number  of 


TRICHOMKS  IN  PURE  SPECIES  AND  HYBRIDS.  59 

cells  than  normal.  All  of  these  abnormal  trichomes  were  found  on  the 
veins  of  leaves,  but  were  not  very  abundant.  An  additional  form  of  ab- 
normal trichome,  which  was  plainly  different  from  any  trichome  to  be 
found  on  the  Juglans  hybrid,  and  pure  lines  as  well,  was  found  in  F2  of  both 
hybrids.  This  type  had  a  stalk  2-ranked  in  place  of  1-ranked,  as  usually 
is  the  case.  The  head  also  was  2 -celled  and  not  4  or  more  celled,  as  in 
the  other  multicellular  trichomes.  This  type  had  its  origin  in  a  trichome 
rudiment  which  is  divided  at  first  longitudinally,  in  place  of  transversely, 
as  is  the  case  in  the  other  trichomes.  From  this  circumstance  alone  we 
here  observe  the  beginning  of  a  new  type  of  trichome,  not  by  the  modifi- 
cation of  a  preceding  form,  as  is  the  case  of  the  6-celled  and  the  8-celled 
trichomes  above  mentioned,  and  probably  the  other  multicellular  trichomes 
also,  but  by  a  sudden  change  in  the  sequence  of  cell -division. 

Numerous  measurements,  which  were  made  on  the  length  of  the  tri- 
chomes and  the  length  and  the  diameter  of  the  heads,  showed  that  there 
is  considerable  variation  in  size,  and  further  study  revealed  a  definite 
relation  between  this  variation  and  the  position  occupied  by  the  trichome, 
or  the  age  of  the  member  bearing  it.  By  keeping  these  facts  in  mind, 
just  comparisons  could  be  made  between  trichomes  placed  in  analogous 
locations  and  under  similar  conditions,  which  otherwise  would  not  be  pos- 
sible. The  general  facts  of  the  variation  as  induced  by  such  environ- 
mental conditions  may  be  expressed  briefly  thus:  the  trichomes  and  the 
heads  of  trichomes  which  are  situated  on  the  veins  of  the  leaves  or  in 
their  immediate  vicinity  are  usually  larger  than  trichomes  of  the  same 
sort  which  occupy  a  position  between  or  relatively  remote  from  the  veins 
of  the  same  leaf;  trichomes  on  young  members  are  usually  larger  than 
those  on  members  that  are  old;  trichomes  from  the  dorsal  surface  are 
usually  larger  than  the  corresponding  trichomes  from  the  ventral  surface 
of  the  same  leaf. 

As  regards  reversion  to  ancestral  characteristics,  the  trichomes  of  such 
of  the  first-generation  hybrids  as  were  studied  were  intermediate  in  size  and 
hence  did  not  exhibit  reversion,  and  those  of  the  second  generation  were 
not  uniform  in  this  particular.  It  is  probable  that  study  of  a  large  number 
of  plants  of  the  second  generation  will  show  alternative  inheritance.  In  the 
second-generation  hybrid,  in  which  nigra  enters,  the  disk-shaped  and  the 
long  secreting  trichomes  both  show  nigra  characters  and  may  be  regarded 
as  re  verting  to  that  pure  species,  while  as  to  these  trichomes  in  the  hybrid 
in  which  regia  enters  the  inheritance  is  not  so  clear.  In  this  case,  the  disk- 
shaped  trichomes  of  the  hybrid  are  intermediate  in  size  between  those 
of  calif  arnica  and  regia,  while  the  heads  of  the  short  secreting  trichomes 
have  the  form  of  those  in  regia,  but  are  much  larger.  Should  the  short 
secreting  trichome  of  the  Juglans  calif ornica  X  Juglans  regia  be  considered 
a  reversion,  as  probably  is  the  case,  we  here  have  another  indication  that 


60  HEREDITY   AS    ILLUSTRATED    BY    TRICHOMES. 

a  trichomal  system  should  not  be  taken  together  as  a  single  unit,  but  should 
be  separated  into  as  many  units  as  there  are  trichome  types.* 

The  hybrid  plants  of  the  second  generation  which  were  taken  for  study 
were  among  the  largest  and  among  the  smallest  of  all  of  this  generation, 
and  all  were  of  the  same  age  and  apparently  had  had  an  equal  chance  in  life. 
The  opportunity  was  thus  given  to  observe  the  relation  between  vigor  of  the 
plants  as  a  whole  and  the  size  of  the  trichomes  which  they  bore.  The 
results  are  contradictory,  and  because  of  this  fact  the  value  of  the  trichome 
in  a  stiidy  of  this  character  is  shown  to  be  great,  as  it  probably  is  less 
influenced  by  conditions  attending  general  plant  development  than  other 
organic  systems  of  the  plant.  The  disk-shaped  trichomes,  and  the  short 
secreting  trichomes  also,  of  plant  4  were  somewhat  larger  than  those  in 
the  larger  plant  numbered  1  of  the  hybrid  Juglans  calif  arnica  X  Juglans 
nig-ra,  while  these  trichomes  in  plant  A  and  in  plant  C  of  the  other  hybrid, 
like  and  like,  were  practically  of  the  same  size. 

*See  page  u. 


COMPARISON  OF  TRICHOMES  IN  PURE  SPECIES  AND  HYBRIDS.  61 

CONCLUSIONS  AND  RESULTS. 

The  11  different  species  and  the  hybrids  derived  from  them,  which  were 
employed  in  the  course  of  the  investigation,  represent  an  interesting  range 
of  habits  and  life  conditions  as  well  as  types  of  variation  and  reversion. 

The  pure  species  are  both  arborescent  (.Jiiglans)  and  herbaceous.  The 
herbaceous  plants  are  annuals  {Papaver  somniferum,  Solanum  villosum), 
biennials  {Oenothera  lamarckiana,  Oenothera  cruciata) ,  or  perennials  (Sola- 
num guinense,  Papaver  orientale,  Papaver  pilosum) . 

The  feature  of  fertility  or  sterility  of  the  hybrids  is  as  follows:  Those 
wholly  fertile  are  Solanum  villosum  X  Solanum  guinense  (5  generations 
have  been  observed);  Oenothera  lamarckiana  X  Oenothera  cruciata  (3  and 
more  generations  have  been  studied  with  regard  to  this  characteristic); 
Juglans  calif ornica  y^  Juglans  nigra.  The  wholly  sterile  hybrids  are  Papaver 
somniferum  X  orientale.  Juglans  californica  X  Juglans  regia  and  Papaver 
somniferum  are  partly  fertile. 

Several  types  of  reversion  are  known  among  the  hybrids,  so  far  as  this 
feature  has  received  particular  study,  and  further  observation  would  prob- 
ably reveal  still  other  reversion  characteristics. 

Two  hybrids,  Oenothera  lamarckiana  X  Oeno thera  cruciata*  and  Solanum 
guinense  X  Solanum  villosum,  are  fixed  forms  in  the  first  as  well  as  the 
succeeding  generations,  so  far  as  observed.  In  the  Oenothera  hybrid 
there  is  a  mingling  of  the  parental  characters,  no  new  character  appearing, 
but  the  Solanum  hybrid  exhibits,  in  the  fruit,  a  condition  not  found  in 
either  pure  parental  species.  The  Papaver  and  the  Juglans  hybrids,  first 
generation,  are  extremely  variable  as  regards  the  leaf -characters.  The 
type  of  reversion  \\\  Juglans  hybrids,  of  the  second  and  third  generations, 
has  not  been  closely  studied,  but  may  be  Mendelian.  The  Mendelian 
characteristic  of  dominance  was  not  observed  in  any  hybrid. 

Two  general  forms  of  trichomes  were  seen  in  nearly  all  of  the  hybrids 
and  the  pure  species — living  forms  (which  are  glandular)  and  non-living 
forms.  After  some  study  it  was  learned  that  the  living  forms  of  trichomes 
had  more  of  interest  for  the  subject  in  hand,  so  that  for  the  most  part  this 
paper  reports  the  behavior  of  such  trichomes.  In  the  Oenotheras  the 
glandular  trichomes  are  unicellular,  but  in  the  other  species  they  are  mul- 
ticellular,  and  in  certain  of  them,  particularly  in  Jiiglans,  the  develop- 
ment of  the  trichomes  was  followed  and  was  found  of  value  for  comparative 
purposes. 

As  repeatedly  observed  during  the  course  of  the  study,  the  trichomes 
were  found  to  present  considerable  range  in  size,  which  is  meant  to  apply 
to  any  type,  and  this  was  found  to  be  the  case  both  in  pure  lines  and  in 
hybrids.  The  extremes  in  variability  were  no  more  marked  in  the  latter 

*Mutations,  Variations,  and  Relationships  of  the  Oenotheras,  by  MacDougal,  Vail, 
and  Shall.  Carnegie  Institution  of  Washington  Publication  No.  81,  1907. 


62  HEREDITY    AS   ILLUSTRATED    BY   TRICHOMES. 

than  in  the  former  case,  which  was  somewhat  unexpected,  inasmuch  as  in 
certain  instances  {Juglans  californica  X  Juglans  nigra,  for  example)  the 
leaves  of  the  hybrid  far  exceed  those  of  either  parent  in  the  range  of  their 
variability.  The  extreme  variability  of  the  leaves  of  the  hybrid  referred  to 
was  estimated  at  500  per  cent,  which  has  reference  to  size  merely;  if  other 
characters  were  measured  the  range  would  be  found  quite  as  great,  while 
the  extremes  in  size  of  the  trichomes  of  these  leaves  fall  under  100  per 
cent  and  usually  much  below  that  figure. 

A  comparison  of  the  trichomes  shows  in  general  that  there  is  a  direct 
connection  between  the  size  and  the  condition  under  which  they  are  placed. 
For  example,  trichomes  which  are  located  on  or  close  to  the  veins  of  a 
leaf  are  larger  than  those  of  the  same  kind  between  the  veins  of  the  same 
leaf,  and  also  trichomes  of  the  dorsal  surface  are  usually  larger  than  those 
on  the  ventral  surface.  The  difference  in  the  size  of  the  entire  trichome 
extends  also  to  the  portion  active  in  secretion — the  terminal  cells  of  the 
multicellular  types.  Owing  to  such  relation  between  size  and  position  oc- 
cupied by  the  trichomes,  it  is  probable  that  the  variation  is  to  be  associated 
with  differences  in  physiological  conditions,  as  nutritive  relations,  quite 
as  in  the  fluctuating  variability  of  larger  plant  organs. 

In  general  it  was  determined  that  each  species  has  trichomes  which  in 
form  and  in  size  are  characteristic  of  the  species,  and  should  it  chance,  as  is 
usually  the  case,  that  both  parental  lines  of  a  hybrid  have  trichomes  type 
for  type  the  same,  but  differing  only  in  size  and  form,  the  corresponding 
trichomes  of  the  hybrid,  at  least  of  the  first  generation  and  frequently  in 
later  generations,  hold  some  degree  of  intermediacy.  But  if  one  pure 
line  bears  trichomes  unlike  those  of  the  other  pure  line,  the  odd  trichomes 
are  transmitted  unchanged  either  in  form  or  in  size.  The  Solatium  hybrid 
was  the  only  clear  case  of  unilateral  inheritance,  although  Juglans  cali- 
fornica X  Juglans  nigra  almost  surely  has  this  type  of  inheritance  also. 

Not  only  do  the  trichomes  vary  in  size,  but  certain  observations  indi- 
cate that  they  have  an  unequal  distribution,  so  that  if  in  size  and  form  of 
trichomes  the  inheritance  may  be  said  to  be  bilateral  it  may  no  longer 
be  considered  such  when  the  facts  of  distribution  of  the  trichomes  on  the 
members  bearing  them  is  taken  into  consideration.  This  condition  was 
especially  noted  in  Oenothera  hybrid.  In  the  hybrid  and  in  the  pure  parents 
the  trichomal  distribution  was  observed  to  be  as  follows:  The  pear-shaped 
trichomes  in  Oenothera  lamarckiana  was  found  on  leaf,  on  stem,  and  spar- 
ingly, on  capsule;  in  Oenothera  cruciata  the  same  trichome  type  was  on 
the  leaf,  stem,  and  abundantly  on  capsule;  in  the  hybrid  it  occurs  on  leaf 
and  stem,  but  not  on  the  capsule.  The  club-shaped  trichomes  in  Oenothera 
lamarckiana  occur  on  leaf,  stem,  and  capsule,  but  in  Oenothera  cruciata  and 
the  hybrid  this  type  is  to  be  found  only  on  the  capsule.  This  observa- 
tion, and  others  also,  make  it  probable  that  the  trichomal  system  is  not 


COMPARISON    OF   TRICHOMES    IN    PURE   SPECIES   AND    HYBRIDS.  63 

to  be  taken  as  a  single  unit,  but  rather  that  it  is  to  be  conceived  as  being- 
comprised  of  as  many  units  as  there  are  distinct  types. 

In  most  of  the  plants  studied  it  was  not  practicable  to  study  the  origin 
and  development  of  the  trichomes,  either  from  a  lack  of  suitable  material 
or  because  the  trichomes  were  unicellular.  But  in  Jug  lam  the  material 
for  study  was  the  most  favorable,  so  that  the  ontogeny  of  the  trichomes 
was  followed  to  a  certain  extent  in  all  plants,  both  hybrid  and  pure  species, 
but  particularly  was  this  done  in  the  second -generation  material.  In 
Juglans  both  unicellular  and  multicellular  trichomes  are  found,  of  which 
4  types  occur  in  californica  and  regia,  and  an  additional  type  in  nigra. 
The  hybrid  strain  with  nigra  blood  has  5  trichome  types,  while  that  with 
regia  blood  has  but  4.  The  trichomes  common  to  all  Juglans  studied 
are  (l)  awn-shaped — unicellular,  (2)  short  secreting,  (3)  long  secreting, 
and  (4)  disk-shaped  types.  The  extra  form,  found  in  nigra  and  its  deriv- 
atives, is  a  long  type  with  a  number  of  cells  in  stalk  and  head.  The  short 
secreting  trichome  has  6  cells,  the  long  secreting  type  has  8  cells,  while 
the  disk-shaped  form  has  32  or  more  cells.  The  6-celled  trichome  has  a 
certain  sequence  in  development  which  it  follows  with  perfect  consistency, 
which  also  is  true  of  the  8-celled  trichome.  The  latter  trichome  up  to 
the  6-celled  stage  has  the  same  sequence  in  its  cell-divisions  as  that  of  the 
smaller  form,  and  to  these  adds  2  others,  which  also  have  a  proper 
sequence.  Therefore  it  seems  probable  that  the  two  trichomes  are  espe- 
cially closely  related,  and  either  that  one  developed  out  of  the  other  or 
that  one  represents  an  arrested  stage  of  development  of  the  other. 

The  first  3  cell-divisions  of  the  odd  type  of  trichome,  that  peculiar 
to  nigra  and  its  descendants,  agree  in  sequence  with  the  corresponding 
stages  in  the  short  and  the  long  secreting  trichomes,  but  in  the  later 
development  it  is  different  from  either.  Only  the  first  2  cell-divisions 
of  the  disk-shaped  trichome  agree  with  the  course  of  development  of  the 
three  trichomes  just  mentioned.  This  type  of  trichome,  consequently, 
probably  is  not  so  closely  related  to  either  of  the  preceding  types  as  these , 
particularly  the  first  two  mentioned,  are  to  each  other. 

In  addition  to  the  more  common  form  of  trichomes  in  Juglans,  abnormal 
forms  and  one  aberrant  type  were  observed  both  in  the  pure  lines  and  in 
the  hybrids.  The  abnormal  trichomes  were  very  evident  modifications  of 
the  prevailing  types;  that  is,  they  were  short  secreting  trichomes  with  an 
extra  cell  in  the  stalk,  long  secreting  trichomes  with  a  stalk  of  more  than 
4  cells,  or  (in  trichomes  with  nigra  blood)  the  odd  trichome  with  more 
than  8  cells  in  the  head,  or,  finally,  disk-shaped  trichomes  with  a  stalk  of 
3  cells.  The  aberrant  trichome  was  observed  in  the  second-generation 
material  of  both  hybrid  strains.  It  had  a  structure  quite  different  from 
that  of  the  other  multicellular  trichomes  and  it  originated  in  a  manner 
peculiar  to  itself.  It  therefore  is  taken  to  represent  a  new  type  of  tri- 
chome, and  is  with  little  doubt  a  mutation. 


64  HEREDITY   AS   ILLUSTRATED   BY   TRICHOMES. 

From  a  study  of  the  development  of  the  trichomes  vs\  Juglans,  as  just 
summarized,  it  would  appear  that  there  may  be  at  least  two  ways  they 
may  take  their  origin.  They  either  may  be  modifications  of  types  already 
existing",  as  illustrated  by  the  abnormal  forms  and  indicated  by  a  com- 
parison of  the  development  of  the  trichomes,  or  they  may  arise  suddenly 
through  the  circumstance  that  the  initial  cell-division  is  a  unique  one.  In 
addition  to  the  origination  of  the  trichomes,  in  the  manners  described, 
observations  suggested  that  at  least  compound  unicellular  trichomes  might 
arise  in  quite  another  manner. 

In  most  of  the  Juglans  studied  the  awn-shaped  trichomes  occur  singly, 
but  in  a  few  instances  they  were  aggregated  into  small  groups,  the  elements 
of  which,  however,  were  quite  the  same  as  the  single  trichomes.  In  such 
cases  it  is  evident  that  the  more  complex  trichomes,  referred  to  in  fore- 
going descriptions  as  stellate  trichomes,  owe  their  origin  to  the  fact  that 
several  adjacent  epidermal  cells  all  give  rise  to  awn-shaped  types,  and 
that  therefore  they  can  not  become  separated  in  the  subsequent  develop- 
ment of  the  leaf.  The  primitive  types  of  trichomes  of  Juglans  may,  there- 
fore, be  conceived  of  as  being  the  awn-shaped  trichome,  in  addition  to  the 
short  secreting  trichome  and  the  aberrant  type.  The  possible  relationships 
of  these  types,  and  their  relation  to  the  other  trichome  \rv  Juglans,  as  con- 
ceived from  their  study,  is  graphically  shown  in  fig.  21. 

Abnormal          Long  secreting 

trichome  In 
nigra  only 


t 


Long  secreting 
trichome 


t 


Unicellular        Aberrant  Short  secreting 

trichome  trichome  trichome 

FIG.  21.— Probable  origin  and  relationship  of  the  trichomes  of  Juglans. 


SUMMARY.  65 

SUMMARY. 

(1)  The  trichomes  of  the  following-  hybrids,  and  of  the  pure  parental 
species,    were   passed  under   observation:  Juglans  calif  ornica  X  Juglans 
nigra,  Fl  and  F2;  Juglans  calif  ornica  X  Juglans  regia,  Ft  and  F2;  Oenothera 
lamarckiana  X  Oenothera  cruciata,  F2  and  F3;  Papaver  somniferu m  X  Papaver 
orientate,  F,;  Palaver  somnifertim  X  Papaver  pi  lo  sum,  Fjj  Solatium  villosumY. 
Solatium  guinense,  Ft,  F2,  and  later  generations . 

(2)  The  Oenothera  hybrid  inherits  characters  from  both  pure  parents, 
but  does  not  revert  to  either  line. 

Three  types  of  trichomes,  all  of  which  are  unicellular,  are  found  in  both 
parents  and  in  the  hybrid  Oenothera.  Those  of  lamarckiana  are  larger  than 
those  of  cruciata,  while  the  corresponding  trichomes  are,  in  the  hybrid,  of 
an  intermediate  size.  Both  in  the  hybrid  and  in  the  pure  parental  lines 
the  trichomes  which  occur  on  or  in  the  close  vicinity  of  the  veins  of  the 
leaf  are  larger  than  those  between  the  veins  of  the  same  leaf. 

The  distribution  of  the  trichomes  is  unlike  in  the  two  pure  lines  and  in 
the  hybrid.  In  lamarckiana  both  types  of  glandular  trichomes  are  found 
on  stem,  leaf,  and  capsule;  in  cruciata  the  pear-shaped  trichome  occurs  on 
the  same  regions,  but  the  club-shaped  trichome  is  found  on  the  capsule 
only.  In  the  hybrid  the  club-shaped  trichome  only  occurs  on  the  capsule, 
in  this  particular  exhibiting-  a  cruciata  character,  but  no  other  type  of  tri- 
chome is  found  on  the  capsule  in  which  the  hybrid  is  different  from  either 
pure  parental  line.  The  peculiar  quality  of  distribution  is  held  to  indicate 
the  independence  of  each  type  of  trichome  as  a  character. 

(3)  Two  strains  of  Papaver  hybrids  were  examined,  both  of  which  were 
sterile,  so  that  the  first-generation  plants  only  were  available. 

Only  one  type  of  trichome  is  found  in  Papaver,  both  in  the  hybrids  and 
pure  species,  which  has,  in  the  hybrids,  an  intermediate  structural  character. 

(4)  In  the  Solanum  hybrid  and  pure  species  2  types  of  trichomes  are 
found,  but  of  these  one  type  only  is  common  to  both  parental  lines  and 
the  hybrid.     The  second  type  occurring  in  the  hybrid  is  inherited  from 
the  guinense  line. 

Although  as  reg-ards  the  trichomal  character  the  Solanum  hybrid  has 
unilateral  inheritance,  in  another  regard  it  possesses  a  new  characteristic, 
the  flavor  of  the  fruit,  which,  in  all  the  material  examined,  was  seen  to  be 
a  constant  character. 

(5)  In  Juglans,  pure  species,  4  types  of  trichomes,  3  of  which  are  mul- 
ticellular  and  glandular  and  1  unicellular  and  lifeless,  are  found  in  each 
species.    Nigra  and  nigra  derivatives  have,  in  addition,  a  form  peculiar  to 
themselves.    Each  form  of  trichome  has  a  manner  of  development  to  which 
it  adheres  with  great  constancy.     Numerous  measurements  show  that  the 


66  HEREDITY   AS   ILLUSTRATED    BY   TRICHOMES. 

trichomes  of  each  pure  species  have  a  characteristic  size  and  form,  but  that 
a  considerable  range  of  variation,  which  is  always  associated  in  a  very  defi- 
nite manner  with  the  position  occupied  on  the  leaf,  or  with  the  age  of  the 
leaf,  is  to  be  found  both  in  form  and  in  size. 

The  first  and  the  second  generations  of  \h&  Juglans  hybrids  were  examined . 
The  first-generation  hybrids  of  both  strains,  as  regards  the  leaf -char- 
acters, are  intermediate  although  not  strictly  so.  Dominance  was  not 
observed.  Each  hybrid  bears  all  of  the  trichomes  found  in  both  parents, 
which  in  the  nigra  derivative  is  one  more  than  in  the  other  hybrid,  owing 
to  the  fact  that  pure  nigra  has  one  more  type  of  trichome  than  occurs  in 
regia  or  calif ornica. 

So  far  as  followed  each  form  of  trichome,  type  and  type,  had  the  man- 
ner of  development  which  was  seen  for  it  in  the  pure  lines.  In  size  and 
form  the  trichomes  are  intermediate  between  the  same  characters  of  the 
trichomes  in  the  pure  lines,  and  also  in  the  hybrid  they  exhibit  a  varia- 
tion directly  associated  with  the  position  which  they  occupy  on  the  leaf 
or  with  its  age. 

The  second-generation  hybrids  of  Juglans  do  not  exhibit  reversions  in 
gross  leaf -characters  to  either  pure  line.  The  course  of  development  of 
trichomes  in  both  strains  was  followed  closely  and  it  was  learned  that  each 
trichome  type  has  the  same  sequence  of  cell-division  in  development  as 
was  seen  for  the  particular  trichome  in  F,  and  the  pure  lines.  The  6-celled 
trichome  has  a  certain  form  of  development,  which  the  8-celled  type  fol- 
lows exactly  up  to  the  6-celled  stage  and  then  it  adds  2  divisions  peculiar 
to  itself.  The  other  multicellular  trichomes  agree  with  the  sequence  of 
these  divisions  in  the  early  stages  only. 

In  the  hybrids  of  the  second  generation,  both  strains,  abnormal  trichomes 
were  seen.  These  were  very  evident  modifications  of  types  already  exist- 
ing. One  aberrant  type  also  was  found,  which  was  quite  different  from 
any  occurring  commonly  on  the  leaves.  The  aberrant  trichome  originated 
by  divisions  which  were  essentially  different  from  those  of  the  usual  tri- 
chomes, and  there  are  no  intermediate  forms  between  the  aberrant  trichome 
and  the  other  types,  from  which  facts  this  type  is  held  to  be  a  mutation. 

(6)  The  trichomes  of  Juglans  are  thought  to  arise  from  three  types, 
namely,  the  awn-shaped,  the  6-celled  (short  secreting),  and  the  aberrant 
trichomes.  By  the  conversion  of  all  of  the  epidermal  cells  of  a  group  into 
awn-shaped  trichomes,  stellate  types  arise;  by  the  processes  of  arrested 
development,  or  the  fixation  of  minor  variations  (such  as  the  abnormal 
trichomes),  or  by  mutation,  the  multicellular  trichomes  have  originated. 
In  Juglans  the  largest  number  of  types  of  trichomes  seem  to  have  arisen 
by  the  second  and  third  means.  The  aberrant  form  represents  a  type  which, 
by  such  variation,  would  be  the  ancestor  of  still  other  kinds  of  multicellular 


SUMMARY.  67 

trichomes.  Should  each  kind  of  trichome  represent  a  distinct  character, 
as  seems  possible,  we  have  here  several  methods  by  which  unit  characters 
may  take  their  origin. 

(7)  \njuglans  there  is  no  direct  relation  between  the  size  or  vigor  of 
the  leaves  or  plant  and  the  size  of  the  trichomes,  although  position  on 
the  leaves  directly  affects  size  and  form  of  the  trichomes. 

(8)  In  the  second  generation  of  Jiiglans  calif  arnica  X  Jug  fans  nigra  the 
short  secreting"  trichome,  in  size  and  form,  reverted  to  nigra.     No  other 
case  of  reversion  to  one  pure  line,  of  a  character  of  trichome  common  to 
both  pure  lines,  was  observed  \njuglans. 


•A 


MAR?    1985 

REC'DMAR  8     1985 


2106  00254  6734 


